After running 802.11n OpenWRT on cheap-ish routers for many years (Netgear WGT634U), I started wanting to just run Debian, and was trying a bunch of different Alfa adapters. Supposedly really good chipsets & most would go into AP mode but some had quite frank limits (realtek would reject clients after the 7th), or reset semi-regularly even with really really well conditioned usb hub power. I spent probably 3 years trying to make it work & accepting some truly forsakenly bad wifi before finally going back & buying a netgear r7800 for AP.
There's a lot more chipsets these days & it might be worth exploring again. The appeal of being able to add passable wifi APs via usb remains alluring to me. And alas the pcie addin market is also not well suited here either... there's only sparse & expensive exotic gear seemingly targetting APs, like Compex cards.