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The 4th edition codex on Tau was pretty shitty, but they had enough going for them from 3rd (and it was similar enough anyways) that they could do quite well.
5th's huge point-cost change raped'em though, particularly in transport costs and the fact that skimmer is no longer the advantage it used to be, despite still paying for those advantages.
Tau are now outgunned by more armies than they used to, outmaneuvered by more armies than they used to, and a lot of their special 'tricks' now don't work as well (JSJ) or were given to others who do it better or for cheaper. S10 now belongs to others as well, who, again, now do it better (with pieplates) or cheaper. And then there's kill points, which make vehicle drones a death sentence. Its not like our special characters are much good anyways.
Their one big gain in 5e though is that markerlights killing cover is more valuable than ever, and the kill of "consolidate directly into another melee" protects the army from "one bad/unlucky placement = tricked out HQ invulnerably in your lines killing half your army on his own from turn 1" like tricked chaos lords from 4e.
There's a "TODex" (named after the old Tau Online site) floating around that's apparently pretty good though it makes it a high-priced low-numbers army.