My primary Linux instances are on my growing family of Raspberry PIs or equiv (I have a couple of BananaPIs and Libre boards). It's based enough on Raspian which like Henry's line WRT 4.2, is just like Debian only different. Frankly, dealing with those issues when you leave the fold is a huge PITA. The problem for me, I really don't have a choice as I can not run a *BSD on them easily to do what I want to do - which is typically to control HW (like my PiDP's or a some "homecontrol" stuff I have).

(at least) FreeBSD works on Pis (though support will usually lag or be non-existent for various "HATs"). And plan9 has worked for much longer, not to mention it's more flexible. Both have gpio drivers. Writing drivers for plan9 should be far easier.