That's even more pathetically puny. Those things are smaller than childrens .22 caliber rounds.
So? It can penetrate many types of body armor, even the steel heart plate
A .22 can't do that, hell, even a .50 from a DEagle can't penetrate some armor
Hell, the round can easily punch through both sides of the armored helmets soldiers wear
P90 is armor piercing too.

In fact any bullet/cartridge can be made armor piercing. The P90 simply has a more powerful round, bigger gun (more accurate and easier recoil control) and a way larger magazine. The handgun sized MP7 is nothing compared to it.
Yea, anything can fire armor piercing rounds, but few are designed with armor piercing in mind
It's size isn't as much of a factor as you may think. It can still be fired with one hand but is easier to conceal than the larger P90.
Collapsible stock and the folding foregrip help improve accuracy when needed
I'm not going to argue against the neatness of the P90, just that in a firefight I'd rather have a MP7