I have a Magpul SL-K and really like it. Lightweight, solid and reasonably priced. I have a UBR Gen2 on my 12.5" .308 AR, but I haven't completed that build yet.
Not that I disagree with you but the Noveske PDW and Q Honey Badger have a really good track record. For a zombie apocalypse type situation, you want commonly found parts but other than that.. if anything ever happens to your gun is sure Noveske or Q would make it right. Or just buy 2
To play devils advocate... these "proprietary systems" make the OAL gun shorter by reducing the need for the buffer tube altogether. So by not limiting your options with the bolt, you're limiting your options with OAL length. A 7.5" Noveske PDW (or any other style buffertubeless system) is going to be shorter than any standard buffer tube style AR you can build.. guaranteed. If you don't mind the extra length, no worries. Not disagreeing with you, just adding another perspective.
Very true. All depends on the intended use. Keep in mind most of the stocks that have a super short collapsed LOP also have a shorter than normal extended LOP. If you happen to be 6'7" with arms like an orangutan like myself that's something to take into consideration.