From what I remember when trying to run FreeBSD (IIRC 11.2-RELEASE and 11.3-RELEASE) on my T-1000 the sparc64/ultraSPARC project is pretty much the work of very few individuals with very limited (and obviously declining) resources. It mostly works/worked, which is nice, but if you expect to "just use it" like on any Tier-1 platform, you shouldn't use it. There are a lot less packages and ports available (IIRC anything which needs e.g. some of those fancy, new languages like rust, node etc) and building something with lots of dependencies from ports can often take days on smaller/older machines. Hardware support was also very much hit or miss...
Due to the declining interest it seems support will be dropped in the 13 branch...
On my T1000 I chose to run OpenBSD, where the sparc64 port except for the "new & easy style OS-upgrade" via syspatch everything is working as expected and installation with various openboot/ILOM/ALOM variants/versions is documented. OpenBSD even has LDOM support and can configure domains via ldomctl and SMP is working (IIRC it was still a WIP for FreeBSD?).
On the package side most packages are available, again with the exception of packages that rely on any fancy new language with a compiler that can't be/hasn't been ported to sparc architecture due to various reasons (usually legal crap or just major disinterest and irrelevance...).
I'm running the T1000 as a firewall/router for testing purposes and have a LDOM set up with some basic networking services to act as fallback for my main router (it is still damn fast as a router, but for my 200mbit uplink the ubnt edgerouter 4 /w OpenBSD is easily capable enough and WAY more energy-efficient...). Haven't yet had a package missing that I couldn't replace with something else. The armv7 tree is missing _a lot_ more packages than the sparc64 port...
NetBSD is known for being ported to anything smarter than your average toaster, so if you want/need even better hardware support and/or larger package repository, you might be better off with NetBSD (I haven't checked/compared available packages for NetBSD vs OpenBSD!). I'm more familiar with OpenBSD than NetBSD, so I'm using it on systems/architectures where FreeBSD doesn't work or fit my needs, but YMMV
