Do you actually play this game or just theorycraft?
Even with Sneak at 1AP, here's how it breaks down...
Build investment: 15 ability points in Sneaking, 1 talent in Guerilla
AP investment: +1AP per attack, because sneaking always breaks after you attack. This varies widely based on the type of attack used, it could means your attacks are using 50% more AP, it could mean they're using 25% more, I guess with special attacks like Flurry it could be as low as 12.5% more, although I'm not sure whether Flurry breaks Sneaking before it hits or not.
Situational: One doesn't simply enter sneak mode. You need to get yourself out of vision first, which often means spending even more AP on movement in order to get outside of vision cones and/or other setup (such as creating smokescreens or inflicting the Blind status). So 1AP is the absolute minimum AP investment involved.
To make it even more situational, missing makes enemies turn around. What this means in practice is: the harder the enemy, the more AP you need to spend to get the Guerilla bonus. This makes Guerilla a "win more" mechanic; it's most efficient against the easier content which you don't really need it for.
In return for all of that, you get double the damage. Now if you're spending 50% more AP to get that double damage (daggers), that's 33% more damage per AP; if using two-handers (least "more AP" for default attacks), that's 60% more damage per AP; if you're spending 12.5% more (which is best case scenario), that's 78% more damage.
Now let's compare that with 15 points in One-Handed (or some other similar weapon ability) and Anaconda as your talent. That's 60% increased damage. Assuming you had 25% in increased damage beforehand (say, 15% from Leadership and 10% from a +1 boost), that's 48% more damage (in other words, a 1.48 multiplier). It doesn't increase your AP expenditure at all, and it's not situational at all.
The comparison comes out roughly equal. Although Sneaking doesn't really get its big payoff until it hits rank 5, once it does it provides roughly the same benefit as an ordinary, boring weapon talent. That's not overpowered at all.
If there is anything exploitable when it comes to sneaking, the only thing it could be is: having an easier time maxing out Sneaking using bonuses from gear. It's normally difficult to track down items with +1 Bow or +1 Two-Handed (and especially +1 T-ability), but it's much easier to get +1 Sneaking on multiple items. There is a chance the item affix is too available. However, I would view such an occurrence as an itemization issue, not as fundamental Sneaking balance issue.
You accuse him of not playing the game, yet your whole post reeks of theory crafting and not actual experience. Getting sneak to 5 is extremely easy with just 2-3 point invested, as the rest is easy to obtain on gear and from traits. Apart from early levels, getting into position behind enemies and outside of their vision cones is not a problem, as you should be having more than enough AP to go pretty much wherever you please, especially on light armoured characters.
Missing an enemy is also not a problem, as spells such as Bless exist, leadership bonus is pretty nice as well. Furthermore, any incapacitating effect on the target makes you hit 100% of your hits anyway.
Even if you do miss, repositioning behind again is not a problem, or if not possible, moving to another target or just moving out of the current vision cone and stealthing is also extremely easy.
Furthermore, the +1AP cost is in many situations a non factor, especially on larger weapons that cost more AP to attack with, as you would not be able to hit 3-4 times in a turn anyway and adding that 1 extra AP cost for double damage doesn't actually affect the number of hits you would inflict in a turn.
The point is, sneaking is extremely easy and extremely strong, while not requiring any real investment. Spending a talent to get guerilla doesn't seem that bad either, since around 4-5 talents seem to be the max that actually have any impact on your chosen playstyle of your character, unless you went hybrid. As a real example, in my game I hit a breakpoint at around level 12ish, when I had all the talents that improved my characters, and anyting else was improving an area that I had not invested anything in, thus being useless to me.
