I wonder why would anyone spend time on developing and maintaining that. FreeBSD needs Linux userland and syscall support to get roughly the same level of softw

I wonder why would anyone spend time on developing and maintaining that. FreeBSD needs Linux userland and syscall support to get roughly the same level of software compatibility. The opposite isn't true (maybe it is for your special snowlake internal application developed specifically for FreeBSD, but it isn't so in general case). All other things you have mentioned are possible with pretty much any of the Linux container runtimes. systemd-nspawn is just one of them, and I don't think it is actually used very much compared to the alternatives.