I have a folding bike, KHS T3 (20") and a road bike Fuji Finest RC, so I can say something about the handling. Especially that I have riding on a folder over several hill routes (to 3275 m in altitude).

The steering in a folder is more sensitive than in the bikes with big wheels (26" or 700c). You can ride a regular bikes with hands off easily but not quite so with a folder (unless your balance skill is quite good).

When riding a folding bike on a steep hill (say 8-15%) and the speed is low, I usually have to put some effort on steering to keep the bike straight. And the standing climb is also not as stable as with regular bikes.