There used to be a thread on the official forums, but it’s possible all the old AoW3 stuff was nuked after the move to Paradox (since the thread isn’t showing up). This was the thread where the devs discussed stealth limitations (especially as it related to the Partisan specialization):
https://aow.triumph.net/forums/topic/do-guerilla-tactics-or-hide-out-do-anything-worthwhile/
This was my understanding of how it worked back in 2015:
(As far as I’m aware, none of the following has changed in a recent patch, so it should still apply) As for Partisan, due to the way invisibility and stealth work vs AI in AoWIII, the entire focus of this specialization is more or less wasted everywhere but player vs player. In the most basic sense, it comes down to how the AI is allowed to see a player’s invisible units in order to compute pathing and and such–in addition to several other decisions that need to be made regarding taking and defending keeps and such. They’ll also see your units for several turns if you’ve moved across their field of view, even if you duck into a tile you’re invisible on and continue to move through such tiles. I don’t know the specifics of every single rule, or what and when it can see what it sees, but suffice to say, the partisan tree relies so heavily on these features that you’re more or less gimping yourself by choosing it when playing vs the AI because they’ll see right through it. You’ll be spending research for nothing, or at the very best, you’ll be playing a guessing game as to whether or not your moment to moment efforts are effective or in vain. And I personally hate trying to take the time to be sneaky if it’s all just gonna turn out to be in vain.
There are other skills and spells to learn from Partisan which are genuinely useful, but you’ll want to make sure the other schools of magic you choose for that game are such that they’ll be able to do most of the heavy lifting.
EDIT: I’m actually seeing a couple mentions in this thread (from yesterday) posted at the official forums that the Devs have been working on making it so the AI are more restricted in the units they can and can’t see, but at the moment this is just rumor. I don’t know specifics. Depnding on the changes they make (or have made) this would change the viability of several Partisan skills. Just something to watch for.
All of that taken from a post I’d made in the AoW3 thread way back when Golden Realms was released, with the (possibly partially non-functional vs AI) Partisan stealth specialization.
If it turns out they ever did redo the way stealth works in the years since, I will be overjoyed because I’ve always wanted to screw around with Partisan in AoW3 but figured it was only useful in multiplayer due to the issues with Stealth in 2015.