Fiddly, fragile (one of my FMA teachers actually did a test with a number of common folders, wearing mail hand protection, and found that quite a number of them collapsed or came unlocked and folded on the hand when thrusting, and even among trainers that I've owned, I've found liner locks to fail and close on my fingers during partner practice, especially if any twisting movement occurs) and likely to close on your hand, extremely difficult to open under stress, and often carried in the front pocket which tends to "seal up" when you lower your center of gravity or step backwards or to the side in response to a threat, especially with jeans pockets that are cut horizontally and not vertically (I use this "sealing up" to my advantage when camping/bushcrafting, as you do a lot of squatting. I hate vertical pockets because everything falls out of your pockets the moment you squat. I've had stuff falling out of my pockets left and right wearing fancy outdoor pants, which must be designed by somebody who's never actually worn them outdoors before...).
I've trained to deploy folders under pressure extensively, and it's just really, really difficult and unreliable. I think a lot of people get good at reaching into their pocket and flicking their knife open, and think they're set, but it becomes ten times harder to do when someone is actually coming at you with their own knife. In such training, I've honestly had far more success just responding empty handed, or with other handier implements, such as throwing my hat in the attacker's face, or accessing a pen in my front shirt pocket.
Another thing to consider is that knives have a somewhat narrow potential role in self defense. The only time, as I understand it, that you would be justified in using one is in response to a lethal threat. Most of the time, that lethal threat is going to be a fire-arm. Occasionally, it might be another knife. Either way, you're looking at knife versus knife, or perhaps more likely, knife versus gun, and neither of those are particularly appealing prospects. If you're going to carry a lethal weapon, a fire-arm is a much more prudent choice.
The damage that knives do also just look really bad. It may be irrational, or unfair, but I think juries are much more friendly to the idea that you shot or clubbed someone in self defense rather than cutting them. I admit that I have no evidence to back up this perception, though.
Batons, or fixed blades, fall in-between folding knives and fire-arms, and I would consider more practical, especially if carried in the belt where they can be much more reliably accessed.