They are insignificant. Only faction they can win anything against are Imperial Soldiers, they get wiped by everyone else that cares to attack them (not talking about DoW).
The main blockade in the Tau's plans is that they cannot access the warp, at least they weren't able to. With the new Goddess "watching over" them, that might change. But this is incorrect, the Space Weebs have had some quality victories. Read up on the Great War of Confederation and (a much more bloody and less glorious example) the Damocles Crusade. The Farsight Enclaves, which are enemies of the Empire, had their famous stand against Hive Fleet Kraken in its infancy.
That being said you're not entirely wrong in spirit, the Tau's victories are not nearly as numerous as other races, the most impressive thing about the Tau is how quickly they've evolved and modernized, they're like the fucking aliens from Evolution. Their weaponry alone is incredibly advanced, the typical comparison of their infantry's standard issue pulse rifle vs. the flashlights issues to the Guard. Shame we won't see 'em in RT, an Air Caste diplomat preaching in your ear is far more likely than an Eldar waifu tagging along imo.