I think Su is just as good a mentor as Tenzin, the problem is that Kuvira was going against what Su believed in. Tenzin believed in Korra and they both were working towards the same things, but Korra's arrogance, impatience, and need to fight things made her choose Unalaq over Tenzin. Tenzin took it hard, but didn't go overboard because he focused on his family instead, which was necessary to him.
Su's family was damaged, and she saw Kuvira as the cause and being in the way of that. Opal is busy with the Air Nation, Jr. and Kuvira are taking over the Earth Kingdom, Lin is in Republic City, Toph is hiding in a swamp, Bolin (her daughter's love) is also working for Kuvira, Zaofu is fucked, and the rest of her family is either a) ready to fight, but can't because they can't prove anything and would be going against the UN; Or b) fucking useless, because architecture isn't important when you don't have the funds and art isn't doing much for anyone right now.