I have been using Linux (Debian primarily),FreeBSD, OpenBSD and Solaris for about 15 years now.
IMHO with Linux, generally things just work, drivers, dependencies, X etc. The kernel comes with more features and capabilities and is generally more useful and practical.
In contrast, FreeBSD and OpenBSD take longer to introduce features to the base distribution and kernel.
Case in point, FreeBSD and NetBSD both have raspberry PI support. The GPIO capability on FreeBSD and NetBSD is pretty poor as compared to Rasbian. OpenBSD hasn't even started to port to Raspberry PI yet.
The one thing that OpenBSD has that is much better than all the other platforms is the PF firewall. The variant of PF included with FreeBSD is a fork that
has not kept up with the advances on OpenBSD.
I had hoped with Apple's use of the FreeBSD kernel in OSX and its subsequent acquisition of a lot of very talented BSD folks that there would have been a huge contribution back to the open source community.
FWIW, I don't care too much about supporting 64 CPUs. Both FreeBSD and Linux had at one point BGL issues that affected SMP performance.
However over the last 10 years most Linux distributions have become like the "kitchen sink" in that they throw everything in (both in the Kernel and userland). Installation sizes are incredibly bloated now.
I work with a lot of enterprise sized companies (>1000 servers). I don't see any other FOSS OSes other than Linux. If you interest is widening your scope for work, then IMHO learn Solaris. If you want to have a lot of fun and turn a few hairs grey, try PF on OpenBSD.