Sorry, your questions are just super broad. But Ill try, long but fast and dirty.
Transition from toon boom... many parts are just equivalent, but probably I would familiarise my self with the fundamentals from hand drawn pose to pose and the traditional studio work. Most of the reasons why it looks so technical is because it reflects the animation studio workflow.
I own 2 versions of the expensive flavour of Harmony, I am subscribed to Retas, and a small app called Cacani, used Flipbook and TVPaint in the past. They are all best at something.
How do they compare.. Ill talk about TB and TZ mainly and make a few mentions to the other ones here and there.
TZ and TB are capable of doing very many things (both of them super broad) but they are not the best at all the tasks they can do.
TZ, TB, Retas are oriented to large teams of people, so none of them are super intuitive.
TB's main use is cutout, TZ main use is handdrawn, Retas main use is Hand drawn, TV Paint only does handrawn.
In digital drawing and pose to pose animation (ability to draw keys, breackdowns, inbetweens) is really a matter of taste, I vote toonz since it has a faster workflow (for me).
In scanning Retas wins, then I would say Toonz (It does look a bit primitive but does the job comfortably).
In traditional animation rituals (how to organize things acording to a paper workflow) First Retas then Toonz
In digital animation database handling, TB and TZ are both good, they scale to large teams very very well.
In cutout TB has a much larger toolset, and thats all I can really say, never tried cutout in TZ but my instinct tells me TB rules in that area. The envelope and curve deformers are more flexible than bones and allow great flexibility in the shapes, you can achieve transformations that nearly look as handdrawn.
Coloring is good in all of them, depends on taste. TB and TZ suports color pallets that can be modified live updating all the drawings at once. They all support methods to color multiple frames at once. Toonboom divides the line art and color art in layers and uses invisible vectors to determine where the color should go. It is a prety smart method, but in my opinion a bit slow if you tried something else. TZ or Retas would be my choice for coloring. The later one works with batches of actions which could be uncomfortable, but it is rather good and supports gradient transitions between flat colors.
In terms of inking and line quality is up to taste. Really in drawing quality you should just try what feels better.
For me the Arc shape offered by TZ and Retas is a must. In TB takes just too many clicks.
TB has timeline and X-Sheet, TZ only X-sheet. Up to you to decide but to me toonboom's timeline is not a pleasant thing to deal with, it is nice for users coming from the Adobe world, or the 3d world. But for me it just slows me down.
TB and TZ uses nodal networks to establish both transformation hierarchies and effects/Compositing. Dont have much experience with TZ's effects but so far I clasify them as equal. Toonboom can use Normal maps to generate automatic lighting, wich I think is amazing. Whether if it is possible to do such a thing in TZ I simply do not know. I personally shade by hand so It just depends what you do.
TB and TZ work in 3d space.
Concerning graph interpolation editors both programs have one. they are different and both logical, I feel TZ's editor is a bit more confortable.
Bot programs can generate inbetween automatically, I find Toon boom's method prety clamcy and after triying Toonz I think it can even rival Cacani (wich is specialized in this area).
TB has scripting language but no documentation at all. It is my great heartbreaker with the software. I have a support subscription wich covers help with scripting but not even then you get documents. The program is very flexible with an excellent Nodal system, but scripting is all pretty obscure. TZ is very configurable and now that it became opensource shines.
TB can import static 3d meshes and request a external 3d program to render them on request. I have no clue if TZ can do anything similar but I am sure it can be customised to do so.
What else..? Price.. I detest subscriptions so I will not describe that part. TB Premium is nearly 2k, with upgrades at 1k, or with rights to upgrade if you join support for 300 to 400 a year. Of course the sales people make offers to customers.
Toonz is free, and promised a premium version at competitive price.
Since TZ is free it can scale up nicely as a renderfarm.
Wich one is more technical, well both are, the main issue is that TB targets individuals more than TZ. TZ is more for a studio workflow.
Blah blah blah.. Phew...