My worry with this is that you run the risk of making fatigue an even more important stat than it already is. In addition to facilitating the using of moves and determining how much armour the merc can practically wear, it would now be the determining factor in the merc resisting a very powerful effect. Furthermore, armour also reducing stunning would mean that you get double the value out of a single stat increase, since by increasing one stat you affect two different ways of lowering the chance to stun. Add to this that stunning would be more prevalent and it becomes a necessity to have that stat maxed out as soon as possible. After all, you’d never want to be stunned if you can avoid it. This would also make backgrounds that boost fatigue even better, while making backgrounds that reduce it even worse. At the moment, fatigue is already pretty much superior to a stat like hitpoints or initiative and this change would make this even more apparent.
Another result of increasing stunning would be the need for the rebalancing of a weapon like the club, since it’s current ‘thing’ is that it stuns. Stunning would also become ubiquitous, which is a fundamental change to how the combat currently plays out. You’d switch to fishing for the stun so that you can reduce the number of attackers, while constantly minimising your own chances of being stunned.