There may be a lot to learn but it's a long term investment, the BSDs are indeed structured as traditional Unix, the skills learned don't change.
I've been a NetBSD for 25 years, it's been my primary system for 20. If I had gone to sleep 20 years ago to wake up in front of a modern system it would still all look familiar. If I needed to install on a drive over 2TB sure I'd have to read up on that newfangled GPT stuff, may even need to spend a few minutes reading up on the rc.d system (systemd could learn a lot from it), but the bulk of the system would function just as you expected.
Of course there have been plenty of developments and improvements in that time but not the back and forth breaking changes of other systems. The rate of change may be slower compared to Linux but it's also surer and more considered.