I ran FreeBSD on a T420s basically until it was too old (I replaced it with a T580). The only real problem was overheating, using embedded graphics helped, but it really needed the heatsink cleaned and fresh thermal paste. The T420s seems difficult to reach and since it also got dropped and the screen was cracked I did not bother. I think the plain T420 may be easier to work on. I think I tried NetBSD on it, but only in text mode, I did also use Ubuntu on the internal drive, with FreeBSD on a 1T HD in a caddy that replace the DVD drive.