It's not too much worse than normal, honestly. I think they are the best shoes for fencing. I may be biased.
I think the only thing that normal shoes do that they don't is a minimal amount of protection on top of your foot and abrasion/cut protection. You can get cut easier through them.
The worst foot injury I've had was from someone stomping on my foot while I was wearing normal shoes. I think it broke my foot, but he didn't get away scotch free. The thrusting tip on my blade broke, and I stabbed him in the shoulder around .5" (~13mm) deep. His blade missed, I think?
I was on crutches for awhile...