On the one hand, sneak is a civil ability that can help you steal and complete quests, but it also can be used in combat. So it's a bit of a weird hybrid ability that kind of seems like it shouldn't necessarily be the most amazing combat ability, but it does still feel kind of lackluster.
Guerrilla gives +25% damage bonus to normal attacks made when sneaking, including backstabs and other criticals. Normal attacks do not break sneak. Skills, grenades, special arrows all break Sneaking and do not get the bonus damage. Sneak AP cost increased to 2.
I dunno, this just sounds kind of awkward and unintuitive. You stab someone in the back, I think they'll realize you're there. Basically, it lets you single out isolated enemies with no one watching their back better than before. Some kind of scan behavior by enemies would be even more necessary than it is already. Moreover, I dislike how this pigeonholes sneak into being good for auto-attacks and nothing else.
I actually think it could be more interesting if Guerilla granted 25% damage bonus and also let you bypass armor with single target skills, including statuses effects (perhaps with only a 50% chance they will apply, or something). So sawtooth attack would cause bleeding on fully armored warrior, chloroform could sleep a mage with full magic armor, rangers could knockdown enemies or set them on fire with their arrows, or you could even use decaying touch or chains on a mage. Perhaps difficult to implement and balance right, but I think it would make it much more interesting and cool. Probably would need to let you sneak only once per turn or something to prevent you from applying to many status effects in one turn, but may not be necessary.
Giving Guerilla utility rather than just a damage bonus would solve the issue of either sneak-attacking is worse than just auto-attacking without stealth or its better. In this case, it would sometimes be better to apply statuses, and sometimes you'd just want to the most damage.
