I avoid using steel cased ammo, but do use both brass and aluminum-cased ammo in my HKs.
The primary reason for using aluminum cased ammo is/can be cost----and if you don't reload &/or don't otherwise have a reason to save your brass cases, aluminum cased ammo is, IMHO, a wonderful substitute. Otherwise, in my experience, aluminum---upon firing---does not seem to flex as much as brass does, resulting in the case not sealing the chamber quite as well, with a slightly dirtier gun resulting. As to aluminum somehow damaging a feed ramp-----well, aluminum---and especially the aluminum used in (for example) 9mm and 45 acp cartridges, is quite "soft," and in my view, such should not be of concern.
The only problems I've experienced with aluminum cased ammunition has been in revolvers-----in particular, small "snubbies"----where the bullet crimp is either inadequate or (in some instances) non-existent, resulting (at times) in bullet "jump," such interfering with turning of the cylinder. Of course, YMMV.