The aluminum framed P-38/P1 was designed to use 115 to 125 grain standard velocity ammo.
Where you can get into problems is:
Shooting hotter then standard ammo.
Shooting heavier bullets.
Installing "extra-power" recoil springs. The stronger springs slam the slide shut with too much force, and the impact will crack the frame through the dismount lever hole. That can literally hammer the front of the frame off.
The German military found that on P-1's shot with hotter SMG ammo, or P-1's that were shot A LOT, the frame would wear in the locking block operating ramp in the frame. So, Walther started installing a hexagonal pin in the frame to give the locking block a steel surface to ride on.
They also beefed up the slide.
If you stick to the proper ammo and take care of it, an aluminum P-38/P1 will be good for a lot of shooting.