Uh-oh. Let me jump in before anyone else and point out that it is a 'magazine' not a clip.
Nitpicks aside, the P90 is a radical design that has problems and bonuses. As far as bonuses go, yes it is compact and it has a large ammo capacity. High rate of fire, low recoil, empty cases ejecting from bottom, and excellent armor penetration are key factors.
On the downsides it is designed to be fired tucked in under the arm and sighting down the barrel. Now this is ideal obviously, but in a SHTF scenario where you may have to discharge your firearm from a one hand stance at anything other than the ideal method above, you encounter problems. Now one asks, who fires their gun one-handed? A guy who is carrying a comrade fireman style but still needing to lay down fire, a guy with one arm shot, a guy who is laying down suppressive fire without exposing his whole body from behind cover, etc . . . .
Another downside includes the awkward magazine location - on top of the firearm. Anyone who has had to quick change a firearm knows the ideal location is in the handle of the firearm under the 'hand finds hand' proof. By that I mean that in the absolute dark you can 100% find your left hand with yoru right hand and vice versa. Barring that, having the magazine well directly in front of your hand is the next best step. Of course anyone who trains with a firearm non-stop will be much more familiar, but I am generalizing a bit.
As far a power, thats a relative thing. Would I consider this tiny little round (about .22) to be more powerful than a .45? No way. Will it penetrate better and fly straighter trajectory (as it has very high velocity)? Yes. But that 5.7mm is more likely to punch through the target and not deliver all its kinetic energy to the target, as opposed to the .45 which will deliver much more mass AND deliver the energy to the target. Obviosuly being hit in the head will kill you equally - the .45 wont do it twice as well. But hitting a long bone (like a leg) with a 5.7mm will most likely deflect while the .45 will shatter.
Take all this with a grain of salt. I think they P90's are really cool. And there are people out there with WAY more experience/knowledge than I. Bu that is how I interpret it.