As far as running steel-cased or aluminum-cased ammo is concerned, you aren't likely to harm your HK with it. The aluminum cases are WAY too soft to harm your gun for sure. The steel cases MAY possibly be harder than the brass cases that your gun is designed to run on, in which case a person could make the argument that it will increase wear & tear on your weapon. However, the reality is that the steel they use to make cartridge cases is necessarily a soft alloy. . . MUCH softer than the steel in your HK.
The next consideration might be buildup of bad crud that you don't want in your gun. It is possible that aluminum, being so much softer & more fragile than brass, might rub off and build up inside your gun at a faster rate than brass would, eventually causing issues. I have heard this theory brought up, but in my opinion it is very unlikely to become a serious problem. Keep your gun maintained, and watch for aluminum buildup just to be safe. As far as steel-cased ammo is concerned, there IS a genuine issue to be aware of. . . Some steel-cased ammo (and even some brass-cased ammo) has a lacquer coating on it which can potentially rub off and build up in your chamber (and maybe even your bore) causing all kinds of stickiness and maybe even overpressure (if it builds up in the bore). As long as you are vigilant about keeping your gun clean, maintained, and lubricated, you will be fine. Steel & aluminum-cased ammo might require just a little tiny bit more watchfulness and maintenance than brass-cased ammo does.
The next question goes to HK warranty when running non-brass-cased ammo. HK pistols are specifically designed, optimized, and marketed to run with quality, brass-cased ammo. Using non-brass-cased ammo won't void your warranty. But if your gun has issues running that stuff, HK will consider it to be outside the intended use of the weapon, and therefore will not help you resolve your issues. Same goes if you use your gun to hammer in staples on a barbed-wire fence-post. If it doesn't work well as a hammer, or if by using it as a hammer you break it, HK is going to say that it wasn't designed or sold to be used in that way, and that you are on your own. As long as whatever goes wrong with your pistol is not directly caused by something outside of it's intended use, you warranty is valid. Some people ask, "Well what if I take my pistol apart down to the last little component and then put it all back together again? Will that void my warranty?" The answer is no, it won't void your warranty as long as you didn't do anything to damage your pistol. It is intended to be field-stripped. It is not intended to be user-disassembled down to the last component. If by disassembling it down to the last component, you cause a failure, then again HK will consider it to be outside the intended use of the weapon. So the bottom line is that you can do anything you want with your gun. After all, it is yours. And HK doesn't mind at all. . . even if you use it to hammer staples. . . just don't ask HK to fix your bad if something goes wrong is all.
What about performance & reliability of non-brass-cased ammo in your HK? Well, brass cartridge cases have certain properties & characteristics, and your HK is designed & optimized around these. The gun is just a platform to enable the cartridge. The platform is designed to optimize the performance of brass cartridges. They certainly could have designed it to function very well with all types of cartridges, but then they would have had to make some trade-offs in other areas (tolerances & accuracy comes to mind). One of the many properties of brass is it's elasticity. When a cartridge is fired, the brass case stretches & expands to seal against the chamber walls so that all the exploding gases go to propelling the bullet down the bore. Then the brass cartridge case elastically springs back into shape so that it can be extracted from the chamber (all this at blinding speed). If it didn't spring back into shape, the gun wouldn't run. It would be a one-shot affair because the case would be stuck in the chamber after the explosion. The elasticity of ammo brass is different than the elasticity of aluminum and steel. Brass cases can be reloaded and re-used many times. Nobody reloads or re-uses aluminum or steel cartridge cases. Steel and aluminum cartridge cases are only good for one firing and then they are trash. They are cheap, and they are the mark of cheap, low-quality ammo. Better ammo uses better components. . . better powder, better primers, more uniform bullets, more uniform powder charges, etc., etc. If your HK seems to run reliably with cheap ammo, and that is what you want to use, it is not a problem at all, but performance & reliability may be lesser than with better ammo, in which case don't blame HK.