The Broadcom fullmac cards should be well supported in Linux. I've seen people complain about poor WiFi range and the like on other Macs, but that is almost certainly caused by wrong/mismatched firmware and NvRAM distributed with linux-firmware for those machines. We're going to be using Apple's blobs exactly the same way macOS does, which are tuned to each specific machine and module variant, so radio performance should be identical to macOS.

I wouldn't buy an M1 right now... just because the next generation is likely around the corner :-). But yes, I can't promise all the polish or that every single thing will be upstream, but things should be solidly in the "daily driver" category a year from now. I'd be surprised if there was any non-optional (i.e. excluding accelerators - no idea if anyone cares about the Neural Engine yet...) hardware left without usable support by then. GPU is a big question mark that should become clear in the next month or two as I poke at the kernel side, but honestly I expect solid OpenGL a year from now, at the very least.