My experience with Boomkins is pretty much non-existent as my highest druid was like 21 and I'd leveled him as feral. Flash forward to yesterday when I finished maxing out my last 'real' alt and decided to use my WoD 90-boost** on the druid and level him up to 100. I understand that they have changed considerably over the years and that the most recent version is quite a bit different from the solar-lunar cycle that existed in WoD, but they seem almost too basic at this point. Certain aspects seem similar to an Affliction 'Lock in that you have to apply a bunch of DoTs, but it looks like 100 talent that makes the the bread-n-butter spam abilities refresh their sibling DoT eliminates a good chunk of that management.
I'll wait till I actually get to 100 to pass judgement, but so far I'm not sure which is better as a secondary-DPS : Elemental Shaman, Warlock, or Boomkin. I'm almost certainly going to be mainlining my Arcane Mage and for healing I'll go with the Mistweaver Monk because I just really liked the MoP Mistweaver and don't want to give up on that playstyle entirely.
**Yes I never used the original 90-boost that came with WoD as I was saving it for just the right occasion. I suspect I will hold onto the 100-boost from Legion in a similar fashion.