Lots of hand waving in here
If you mean your Glock 20 can't take 10mm loads that are unsafe you're probably right
If you're saying your Glock 20 cant take Underwood level loads then I'm not so sure about that
Given that I don't reload I would rather have an hk USP expert in .45 so I could use off the shelf ammo. 10mm can be bought but isn't a "Walmart caliber" and while .45 super is obviously even rarer, you can use pretty good .45 ACP loads or order online for .45 super. The bigger issue is that hk hasn't imported usp experts in .45 in a while. There were some elites that went to CA but elites look hideous.