dm-flakey runs inside the kernel so is going to be faster. With nbdkit + NBD loop mounting there's a bit of overhead going through a Unix socket, which is avoided by using ublk + nbdkit plugins, but you still have to go out to userspace for each device access.

The advantage is you can write plugins as regular code in lots of different programming languages, with a stable API and ABI: https://www.libguestfs.org/nbdkit.1.html So it's a lot easier to experiment or write plugins for odd devices.

Also our sparse RAM disk supports huge disks (up to 16 exabytes IIRC, 2^63-1 bytes whatever that is).