I have a pack of Nikon FGs and a couple of M4s. I don't know what the FM3a offers the photographer that the FG doesn't and the FGs cost $50 each. I love them--light, good meters, M and A settings (and P, which I never use), and very ergonomic. If I leave one under a chair somewhere, someone else can have it, and I won't cry. They're a good match for my Leicas in every way, except as the OP notes, everything's cheaper. I like both systems, and tend to use one or the other for months at a time, then switch when I get bored, and I find that it's the switching that keeps me on my toes, not the system I'm using. I think that more likely a system can get in the say, rather than help. I'm the guy who keeps his digital Nikons set on A priority, one central focus point, average metering. The strategy works fine. I feel lucky when a camera even has a meter in it. Maybe I missed something about batteries---is there some reason I want to pay $400 to have a camera that doesn't use a $2 battery, when I can carry a lifetime supply of them in the bottom of my bag?
I don't use anything except from 20mm - 100mm, so fancy lenses don't matter. I have close-up equipment for both systems, when I need it. The Nikons are backed up by digital, which is a point for Nikon (can't afford Leica digital). Differences in viewing systems--one being better that the other for wide angle, for instance, become theoretical after a while--I can make either work fine. I've noticed that I take different photos with either system, and generally think my RF photos are better, but that could be because I started with Leica over 50 years ago and came to SLRs after I had a lot of habits formed--I'm not prepared to say either system is really better.
Anyway, I think a lot of people conflate differences in their toys with personal habits. There are good and bad reasons for any system to work, but ultimately to blame the system for bad photos? . . . . the real problem is in the mirror.