OpenWRT also doesn't use PAM by default apparently, and there is a risk of people using password login, which you should never do past initial setup.

But outside specific niches, musl is rare, with good reason. Performance is a joke compared to glibc, especially the allocator. The only reason to choose musl in my opinion is for memory constrained devices (OpenWRT etc) or for building static binaries where performance doesn't matter but portability does. I consider the "small container" use case questionable: performance will suffer, and you could share large base layers instead which would amortize the cost over many different images.