I've yet to feel legitimately challenged in Spirit Mode. That's not to say I haven't had a hard time, or that it's been smooth sailing as I've traversed the board, but I haven't run into anything that's tested my skills or forced me to adapt how I play.
For anyone who doesn't know, Spirit Mode is Smash Bros. Ultimate's version of a single-player campaign. You start out as Kirby, and you make your way all over a giant map and complete challenges as you go. Some of these challenges unlock a new fighter on the roster to play as, and all of these challenges are some variation of a basic Smash match. You're pitted against a CPU or a team of CPUs and are tasked with taking them down. The twist in this mode is that you can equip Spirits, which are pictures of various characters from other franchises that act as stat buffs and come with abilities or traits, such as increased damage with Magic attacks or the privilege of starting a fight with a certain item. These Spirits can be leveled up to increase their effectiveness in battle, transformed to gain new traits, or trained to infer additional buffs and debuffs.
However, the enemies that you're facing also have these Spirits equipped and the fight is lightly themed on the spirit. For example, a fight against Mario with a Hal Emmerich Spirit turns this to fighting Dr. Mario alongside a giant metal R.O.B. Get it? it's like fighting Dr. Emmerich and a Metal Gear. These are generally fun little nods to characters which couldn't be included in the game, and each and every Spirit battle has a different theme, even if you do start to see repeat effects after a while. There are only so many variations of fighting with periodic screenshake against the Spirit of a large enemy, after all. Once the enemy is defeated, you gain the Spirit for use in future battles.
The issue with this mode is that the idea of difficulty isn't present in a satisfying way. Many of these fights are over in a few seconds if your Spirit is strong enough and you choose the right type from the effectiveness triangle (Shield, Grab, or Attack). Once the battle is done, you pick up a spirit that likely does the same thing as a handful of other spirits you have, and then you move on to the next dot on the map to repeat. Occasionally, though, there is a particularly troublesome battle in front of certain pathways or treasure chests, just to make sure you can't stomp your way around the board with no resistance. These battles almost always have the same conditions: Fight 4 of the same buffed enemy on a stage where there is a negative event that is especially difficult for you.
Cut to you being bounced around the stage helplessly, trying out different item cards between rematches, until you luck out on CPU behaviour or get a lucky hit in. I've "won" some of these battles because CPUs have just run off screen because of an item I was holding, or because they chased a healing item off of a transforming stage.
Fighting 4 Yoshi, fighting 4 Inkling, fighting 4 Kirby, fighting 4 Wii Fit Trainer, etc. with no team damage for CPU and powerful Assist Trophies or stage events (such as earthquakes causing tripping, wind blowing the player away, sticky/lava/electric floors that don't affect the enemy) is not a very fun time. On top of that, having to use fighters that are unlocked sequentially makes progress on these stages difficult. The weaker battles are too easy to force familiarity with any one unlock, but the harder battles are too hectic and frustrating to learn effective strategies from. Sometimes, you have to use up Spirit slots that would have been for items or increased damage just to negate the stage events.
There is a lot of Single Player content in Smash Ultimate, but it's nowhere near the level of Subspace Emissary, which was an actual journey through a unified story mode, complete with cutscenes and character moments. Spirit Mode is a glorified Special Smash that rocks from being a breezy, fun time to becoming teeth-gnashingly unpleasant, forcing you to grind out Spirit cards and use characters you don't enjoy playing, and giving up abilities that might give you an edge in a battle just to get back to neutral. I find myself pushing through it just because it's a reliable way to unlock characters, rather than out of an actual desire to see the end of the mode.
I've never had to grind out spirit cards, the Gym (located in the town near the starting area) levels things up fairly fast and the snacks are plentiful. That being said I have found a lack challenge with most of these barring a few. (that Paulina stage is hellish)
I dont have fond memories of Subspace Emissary I remember having to trudge through it with my friends when we just wanted to play with a full roster against each other and it just dragged on. This by comparison is fun to pick away at and isnt required to enjoy the rest of the game.
I've played about 12 hours of the mode so far and I really like it. I agree it doesn't have the cool story of Subspace Emissary (Sakurai's complaint that the cut scenes get spoiled is just weird, because people still enjoy them and people with a little self control won't let them be spoiled if they don't want them to) and the levels aren't as well designed, but there's so much content and a lot of it is very fun and wild.
I agree that there are a fair number of bullshit frustrating battles in the game, but I don't mind that either. Many of them are optional (either they're at the end of a path or off the main path or you can go around them) and many of them are very manageable if you go find the right spirit or, importantly, character.
And that's something you left off from your description. A lot of these fights seem designed for or around a specific type of character. If you only main a couple you're going to have a tough time. If you're getting swarmed by little dudes you can pick a bigger character with a strong spinning attack and wreck shop. If you're getting pummeled by an overpowered big slow character you can pick someone quick with good projectiles who can ping them to death. You may not have the right guy when you first get to the fight, but a couple hours later you might pick the right one up and go back and smash a fight that beat you a dozen times when you got to it, which is a fun and satisfying feeling.
There's always another path to go down, another direction to take, a different tactic or strategy, something that will see you through. It's like an RPG or even a Metroidvania. If at first you don't succeed go somewhere else and come back when you're better equipped.
As someone only interested in single player this is a bit of a worry. I've been burned once by Nintendo this year when I jumped in on Mario Tennis Aces and absolutely hated the single player which was nothing but bullshit difficulty. Still tossing up whether to jump in on Smash, although at least here it sounds like the worst of the weird difficulty is optional.
Admittedly, the adventure mode can be a bit of a grind at a certain point. However, even if you ignored the adventure mode entirely, the classic ladder mode alone is pretty great with each character having their own themed ladder and some of the unique boss battles from adventure mode being integrated into some of the ladders.
There are some fights in Spirit mode that are just ruthless and a huge pain in the ass, even on easy and are absolutely unfair, but with so much content after a few tries if I feel like I'm not getting any better with this one boss or fight that i just can not deal with, i just move on and do something else in that mode or mess around with the few other single player modes to change things up.
It sounds like you are finding Spirit Mode incredibly easy, pretty opposite of my experience.
With the last Smash I played being melee and as a person who doesn't play fighting games I have found "Hard" to be pretty damn tough. While fight difficulty varied a lot early on, after some considerable time now most of the 1 and 2 star opponents are beatable purely through sheer power (since the power level difference equates to additional damage you output and enemies do) with 1 stars being pretty easy, but there are some frustrating 2 star fights even being thousands of power ahead. Meanwhile there are still numerous 3 star opponents I attempt a few times and then just leave there as I don't even see a sliver of hope in the first attempt. There are fights where I am 1-2 thousand points behind and they feel basically impossible, meanwhile there are numerous fights where I am 1000-3000 points ahead and they also feel almost impossible (mostly multi-opponent or some stamina battles).
Now one thing I will say is the AI on hard seems to vary its level fairly drastically. This makes some fights a complete joke (even though its like 2K power above you) and other fights absolutely insane (even though they are 2K power under you). They don't tell you the level of the AI, but its pretty obvious it varies between like 6-9 and with the various coding changes, power ups, designs like stamina, and multi-enemy battles the obvious level 9's are incredibly frustrating, especially the multi-enemy fights where you have like 4 pretty obviously level 8-9 bots on pretty sub-optimal stages for the encounter that also receive an enhancement. Now I presume these are meant to be the challenges (like in melee there was a challenges mode), but its a bit annoying when the number of no-frills fights are in the single digits (that being said the no-frills fights are by far the easiest).
All of that being said I have found spirit mode enjoyable (especially at the start) with the 2 boss battles I have done (mostly been clearing the map) feeling dark souls like. The first one I did in the bottom left involved me switching up my spirits, trying fighters I normally don't use, and learning to roll like never before as you can take 3-4 hits at most since you have 1 total life unlike in classic. I am pretty excited to further explore the other dungeons and defeat the other bosses.