Yeah, I've found the underpowered Russian stuff to work fine in a good rifle too. I've shot it out of BCM's and an Armalite. On the one BCM, when the rifle was new and still wearing in, I noticed that it wouldn't lock the bolt back on the last shot. As the rifle wore in though and the buffer spring softened up a little, that problem went away. The Russian stuff is also dirtier. It doesn't seem to expand the case to seal the chamber off the same way brass does. I don't know if that's because it's not loaded as hot, or if it's because of steel not expanding and contracting the same way brass does. But it has fed, fired, and ejected just fine in my (relatively limited) experience.
The 7.62x39 also is often limited by its ammo - namely, the cheap Russian stuff. Load an AK with good US production brass cased stuff (or good ammo from a foreign manufacturer) and it's often surprising how much better an AK can shoot. I don't think it'd quite match an AR in general, but still, a good AK can shoot. Also, I owned a VEPR K .223 once and that thing would shoot. Maybe it was the ammo I was shooting, or maybe it was just that rifle... but do I ever regret selling that damn thing and the stack of Circle 10 magazines I had for it!
On the SIG, I think my friend's is a good rifle. He hasn't complained about reliability or accuracy, but it's picky about what magazines it will allow to lock in. It's also not really any better than any of his AK's or his VZ-58. And if you need replacement parts for the SIG, you'd better hope SIG is selling them or someone else has them. If you need parts for an AK, some are available but maybe not. If you need parts for a VZ-58... I don't even know where you'd find them. If you need parts for an AR-15, you'd have to be a howling idiot to not be able to find them.
The AR isn't superior to everything in every way, but taken as a whole package with logistics and ammo availability added in, it's about the best you'll find in this country IMO.