I’ve actually done a few 80% lowers with a drill press, hand router and jigs, they came out well but AR lowers are so cheap now it’s not really worth it in my opinion.
I’m still building my MPCNC so I don’t have any experience with it yet but I would think if you can do one with a hand router it would be somewhat possible to do with the MPCNC. Especially if you milled out the majority with a drill and just did the finishing with the CNC.