Personally, if it doesn’t have a locking mechanism, I wouldn’t try using it in combat. During normal use, you’d never apply pressure against the back of the blade, so the knife wouldn’t be at risk of folding. Additionally, if you were concerned about a risk of the blade folding, you’d grip the handle along the edges, avoiding the groove that holds the blade for storage. But, that’s not really an option in combat.
Larger folding knives usually have some kind of safety catch designed to prevent the blade from collapsing on your fingers while using it. This may be as simple as a bent plate that snaps into position stabilizing and locking the blade while open, or it may be a hinged mechanism that holds the blade in place, and can be released by pressing a control somewhere on the grip.
Normally, you’ll start to see these on blades over three inches, though some smaller knives do include locking mechanisms.
I wouldn’t worry about the blade taking off the user’s fingers, but, having the blade collapse, cutting into the index finger, would be a real concern.
The thing about this is, most pocket knives that lack a locking mechanism aren’t going to be large enough to use effectively in combat. So, the risk of them folding on your hand is fairly slim. Larger knives should lock open. It’s also worth noting that I’ve never seen one of these mechanisms fail, I’m sure it can, but it’s an extremely rare occurrence.
If you end up with a knife large enough to use in a fight, but without a locking mechanism, I’d recommend against using it. At all, really, even as a utility tool, I’d worry about the risk of it folding. These locking mechanisms aren’t there so you can stab someone, they’re a safety feature for using the knife under normal circumstances. A large folding knife without a locking mechanism of some kind just wouldn’t be safe to use.