Okay, after reporting my problems with my Ralink-based wireless adapters to Asus, including their own USB-N66, Asus just sent me a new Beta version to test.
I've done so, and it works great with the Ralink wireless adapters that I have (Linksys AE3000 and the Asus one). Connects first time every time, good wireless speed, seems just fine.
So the fixes for these wireless adapters are coming, I'm hoping in the next official release version, since they have this fixed in Beta now.
I won't be running the new Beta for very long, since I've already gotten the "Neighbour table overflow" errors, and don't have an IPv6 firewall (which I've been working around with boot time scripts that I don't think I can run easily with Asus firmware), I may just turn IPv6 off soon, since it seems to be fine.
I'm sorry, I don't know about the Broadcom chipset wireless adapters, I don't have one of those to test with.