I think the main issue with stealth, for me, is that you can still (?)
1) simply switch an out of combat character into TB-mode and traverse the entire battlefield, and simply navigate past people's vision cones (and now, threat areas?) and they have no passive check to detect you (or has that changed)?
2) as a rogue, stay out of line of sight ,and the threat area, and sneak attack + hide BA as before? Ranged sneak attack wouldn't be fixed by melee threat area, unless they have done something else.
I am speculating, but it sounds like these may still be possible.
I think (1) needs passive (out of turn checks) - Solasta does this (as far as I recall - they have a 'visibility meter' that fills as you sneak closer, and it was much harder to avoid triggering combat, certainly with players in armor)
(2) needs the rogue to be placed in the initiative order when they make an attack (of any kind). Of course, if the AI continues to ignore them, that is not good, but at least they are in the same time regime as all combatants.