unfortunately SiLabs haven’t updated their OS X drivers to make them BigSur friendly yet, and I needed BigSur for another project.
Well this is curious (and probably not usefully reproducible). My N2+ was refusing to boot and re-flashing the eMMC wasn't helping, so I flashed an SDCard and tried that (with eMMC removed). Didn't work. In frustration I flipped the MMC/SPI switch to SPI and back, and powered it back on (with both the eMMC and SDCard installed). Lo'and'behold it booted! So I went through the initial set-password-and-create-user-account process, then rebooted again... after which I had to do the initial setup process _again_. Now it boots as expected -- so for some reason the first time it booted from the SDcard, and the second time it booted from the eMMC (with both flash devices inserted). The docs seem to say it ought to prefer the eMMC if it is present, but apparently it didn't.
Anyhow, haven't gotten any further yet.
I think my residual problem arose because I installed too much while backports was in the apt sources list. By only having it active while I installed clang/llvm 8, and nothing else, I seem to end up in a good state. So far.