I've found that it varies drastically based on what you already have in your deck and merfolk is one of the riskiest/trickiest to draft. Kumena is maybe worth drafting around as is the elite guy that lets you put out a +1/+1 counter every time a merfolk enters the battlefield. If you see a late Merfolk Mistbinder then you might consider going in on it, but then you still want to pick up at least a second one. The problem is, if you open a Merfolk Mistbinder and take it with the idea of going into merfolk then you have to hope that other people open them (and don't take them) as well. Otherwise what are you hoping to get? Drafting a bunch of River Darters (2/3 for 3, unblockable by dinos) isn't where you want to be. Jungleborn Pioneers are much better, but anyone in green is going to take those highly. Most of the good merfolk are in IXA, so you end up hoping to get lucky in the RIX packs to have a good starting point and then hoping to get lucky again in IXA that specific merfolk cards get opened. Like, what if only one River Herald's Boon is opened in all 8 packs?
One of the best decks that I drafted was a merfolk deck. I can't remember if I posted it here, but it had triple Forerunner and in the IXA pack I opened up the guy that gives all merfolk with +1/+1 counters unblockable. But if you think about that - I had to open/get passed 3 premium uncommons and then open a bomb rare in my tribe in pack 3 (I also had at least one mistbinder I think and maybe even an elite). So really I just got incredibly lucky. Think about what that deck looks like if only one of the uncommons was opened (seeing only one of a specific uncommon is pretty normal). There's always luck involved of course, but it just seems like merfolk is super reliant on getting lucky with the downside being an almost unplayable deck.