Finally, I use a NanoPi NEO3 with the Rockchip chip as a NAS. It was previously running on OpenBSD and will soon be updated to NetBSD with a cgd (block encrypted) USB drive.
As for your question, it's not harder to run than, say, Slackware, as it doesn't change much with time. This is the system you choose when you're very conservative with Unix systems and expect something simple, understandable, and predictable in the long run.
As for 'why', it's a matter of personal preference. NetBSD is about preserving the Unix heritage; you could still install it on a MicroVAX II, read filesystems from V7, or run SunOS 2 binaries if you wanted. It still feels much like using 4.3BSD on modern hardware. A good share of NetBSD developers purposefully use old and/or underpowered machines for both development and testing, which keeps the system clean, fast and minimal.
OpenBSD is a fine choice too, but it's relatively slow and lacking in software. For instance, vmd is pretty far behind bhyve and NVMM when you want to run anything more than Alpine VMs. This may have improved recently, though, I haven't checked.
