Tau are the worst army at CC in the game, bar none. Nids are arguably the best in the game, or at the very least the fastest. Fast armies aren't as affected by Tau's main strength, ranged firepower. Nids main strength, CC, is not negated by Tau at all. Yes kroot can be used against CC armies, not because they are all that great, but because they are the only option. Yes you can load up on template weapons, but not enough. If you are going to be fighting Nids a lot, don't bother with Tau. It will just be frustrating, and if you do happen to win, your list will be SO non-competitive against anything but Nids that it just won't be fun.