I don't particularly like the Stormsurge as a model and I wish it weren't a Gargantuan Creature, but it doesn't bother me in general. Nothing in 40K makes a hell of a lot of sense, anyway, so the Tau being the only "sensible" ones seems foolish. I mean, clearly this is a universe where, despite all reason, there is good tactical reasons to put a sword on a walking apartment building loaded with guns. Why shouldn't the Tau make one that has missile arms?
The Rule of Cool is the master of the 40K universe and always has been. If it's cool, then it's fine. Giant mechs are cool and, furthermore, are very Mecha/Anime (which is part of the Tau's whole design style anyway). If anybody should get a giant mech, it's them.