I've run into fun issues like FreeBSDs slab allocator not playing nicely with the ZFS/NFS workload on one specific server and having to drop down to having ZFS allocate memory in a different way that used about twices as much CPU to avoid random multi-second pauses on the whole system. Also had issues where kerberized NFS would get confused and something would crash so had to have random cron jobs to restart dying components, since FreeBSDs init system is so simple. Also arbitrary and short name length limits, I think in both bhyve in the past (couldn't use fqdns for VM names since that would have exceeded the limit) and something about zfs mountpoints ages ago (I think we ended up changing our naming and nested conventions to work around)