I believe the P22's slide is cast, and plus it has to be lighter in weight so it would be less sturdy. But you're not firing centerfire stuff in it, it can be less sturdy. I think there have been a couple of cases of slides cracking around the ejection port from people firing lots of hyper-velocity ammunition, which will wear out any gun sooner.
The P22 is a great trail gun too, very small and light when compared to some other .22's.
If I already had a P99 and wasn't looking for a carry gun, I'd choose the P22.
I usually use minimags, I used stingers and velocitors to break in the gun. Stingers are in it when I'm in the woods. Remington-Peters high velocity lead .22's work well too, but they leave a lot of lead in the barrel. Federal high velocity doesn't work at all in mine.
CCI Blazers work well in mine. Stay away from Thunderbolts. They'll coat your barrel with lead. I didn't have very good luck with the Winchester Xpert 22 hollow points either. They failed to feed on a regular basis. The P22 is a great gun. I actually went the other way, bought the P22 at a gun show, and then got the P99 because I loved it so much.