This is why, going forward, I'm not making changes. I don't know the NPC creation system well enough to make those calls. Still, I bet you need that heavy-hitter Pit Fiend to challenge high-level characters.
The more work I do, the easier it is to do future work. I was surprised how much work I had to do for Tales, but there were so few stat blocks, it was a weekend's work. That's all. Creating the pure version was simple, though I wouldn't be surprised if I made a copy-and-paste error. :-)
New Docs
I've made a few more documents and have one more planned. A complete list can be found at the bottom of this page: https://gsllc.wordpress.com/2017/03/27/5e-dnd-one-stop-stat-blocks-for-tales-from-the-yawning-portal-rpg-bandofmisfits-stitched/. Cutting to the chase, I've created docs...
I incorporated the official Monster Manual errata (http://media.wizards.com/2015/downloads/dnd/MM_Errata.pdf) into the stat blocks and, as promised, corrected the Elite Drow Warrior's shortsword attack to include poison damage.
Oh, damn. <-- let's see if that's replaced with smiley faces.
I knew about covens but didn't notice that when I made the hags. I'll add it as a side bar next to the Green Hag stat block.
I don't own Princes of the Apocalypse and don't plan to buy it, so I won't be able to convert those.