I agree that turn order is currently awful and encourages some strange tactical decisions. Like leaving enemies alive with 20 hp so that I can oneshot them next turn and have 2 of my people go in a row. Or not using totems because they steal places in the turn order from my heroes.
I can give some clarification on the mentioned AoO issue. The AoO issue is known, and the explanation for this issue is rather simple actually: Ai does not take AoO into account at all. This is intentional (we don't want the talent to go to waste), but clearly it's giving some issues at the moment. We have plans to fix the behaviour in the future, which should get rid of the annoyances that have been mentioned (like getting killed by AoO) while ensuring the viability of the talent.
Wait, but it would still be useful if AI took AoO into account. I primarily took the talent to somewhat control AI movement. Like, you teleport enemy archer or rogue to your opportunist and they won't go to high ground/to hit your other heroes because enemies want to avoid AoO. For me that'd be a far more interesting use of the talent than simple "enemies get hit each turn because too stupid to take AoO into account".
And somehow you don't prevent enemies from taking glass cannon or undead into account so why is opportunist treated differently?
EDIT: I think there's a simple way to somewhat fix issues with the turn order while still preventing your whole team from going first turn 1 and murdering everything. Have the turn order established turn 1 be permanent. Summons always go right after the summoner. When an enemy is kill, another won't jump to his place in the initiative queue, instead 2 of your characters go one after another. Wits would still be useless though.
Alternatively, have the new turn order only apply to turn 1 of combat. All the other turns should determine the turn order based solely on initiative.