Playtest #3 using chapter 3 of Light of Xaryxis for testing with 4 level 5 PCs. Had our first character death during the first fight of the adventuring day (4 CR4 and 1 CR2 monsters from Boo's Astral Menagerie), even without monster crits (they had 2 Nat20s for what would have been an additional 22 points of damage). PCs only had one attack crit (lost 7 points of damage). The PC who died had 2 lost death saves from 2 hits from the monster who dropped him (the 2nd and 3rd of a multi-attack) and then rolled a 2 on his save at the start of his turn (didn't have inspiration to use, but would have).
Once again, I'm struck by the thought that MotM and later monsters have been built with the new rule of monsters not critting in mind, resulting in much tougher encounters in general (rather than only if the monsters get lucky with a string of crits in a single encounter). This is the 2nd combat encounter in 3 sessions in which monster crits would have TPKd the party in a group that has 2 eight year veterans of 5e and 2 four year veterans.
The player who had his Changeling Rogue perish in the fight came back with a human wizard, built with the new rules for humans, and used his inspiration on an important saving throw in the new PCs first battle, possibly preventing his second character death of the session.
Will be a couple of weeks before we play again. If there are rules released between now and then we can incorporate we likely will.