How this got off into TorBrowser puzzles me. Presumably the goal for clients is to reach them where they are. Certainly providing a diversity of clients is a "good goal" but beyon dreaching more users I fail to see how it helps the network writ large. I suppose indvitual users woudl be mroe secure with a wider array of clients available, but that's generally true of the Windows monculture in desktop computers. And as we shift to Android moblie devices as people primary internet connection many of those issues will likely follow, but I digress...
For relay diversity it's more obvious. If you can sabotage 93.6% of the band width because a Linux specific bug either in TOR or just the OS that's a huge problem (from https://torbsd.github.io/oostats/relays-bw-by-os.txt, sorry I forget who to credit but if you read the thread you will see...).
I run https://atlas.torproject.org/#details/A53C46F5B157DD83366D45A8E99A244934A14C... which in it's current incarnation is FreeBSD 11 since January 2017 or so. Though for most of it's life had been Debian or Ubuntu.
I'm definitely a "Linux Weenie" specifically of the Debian family so I enjoy what "unattended upgrades" can do interms of when and what you automatically upgrade and when/if to reboot if needed, but really the 98% of that that really matters you can do with a small shell script + cron. Installing TOR isn't really appreciably harder either "pkg install tor" is just as easy as "apt install tor" for people who want to live in a packed rather than source world. So I don't think the complecity barrier is real (though the perception of one may be a deterrent).
Looking at https://en.wikipedia.org/wiki/Usage_share_of_operating_systems#Public_server... it seems TOR relay OS's while skewed toward Linux and away from Windows aren't that far off from at least soem studies of "Internet based servers' market share". I debate the accuracy of these in general but within some largish margin of error they are probably not untrue.
Monoculture isn't just a TOR problem. I think we should advocate for it but I don't think we can solve it just in a TOR context. So yes if you run relays go learn some underserver Operating Systems the BSDs are a little weird at first coming from Linux but you're clever you'll manage or maybe go way out there to one of the Firefly (OpenSolaris) variants...but diversify all the things nto just TOR.
-Jon