I believe that the sig mcx does and maybe a few of the other Piston driven ar conversions but I think you are going to be hard pressed to find anything that is truly as customizable as the ar without a little bit more work. The sig in a standalone upper I think needs a converter to play nice with a milspec lower.
I just bought a Sig MCX. It has a picatinny rail and key mod forend. The stock folds and the butt has an adjustment ability not unlike the standard AR version, though it is Sig proprietary. Much of the internals are Sig proprietary as well. Mine is .300AAC Blackout, so regular 5.56mm. AR mags work.
The stock is folded by pulling up on it, there's a "lip" that holds it and you're working against a rather strong spring. It's held against the receiver when folded to the left, also by that same spring, so it doesn't flop around. When deployed, it seems sturdy enough.
it has QD points on it. In addition the rear of the receiver has QD points on the rear, two opposite each other. The forend on my rifle will need to have adapters with QD points. I have a MCX Rattler pistol which has two QD points on the rear of the forend.
The pistol grip I think can be switched with any AR/M4 grip, although the one on my Sig is very nice and I'm keeping it as is.
The bolt looks very AR-like but the bolt carrier is very different ....almost like the front half of an AR carrier, and it's attached to an overhead mechanism with dual recoil springs, so of course it's hardly compatable with the AR-15 on the inside.
It looks like a very well made rifle, and it's in the $1500.00 range, so it should be. But if you need AR compatability consider the differences I've noted.