Chapotraphouse

Back in 3.5/4th. Edition when they were introduced, they had some really cheesy (a.k.a overpowered) units that broke a lot of the game's fundamental design rules.

For example, they had fast hover tanks with front armor 13 when all previous hover tanks maxed out at 12 (14 was the absolute maximum). This meant their tanks were fast and not far off from having the armor of a Land Raider or Necron Monolith. But they had the same speed as Eldar vehicles. So for each other faction, it was either T'au tanks had the same armor as your tanks but were twice as fast, or they had the same speed but higher armor.

They also had a rule allowing a unit to split its fire between two targets. Again, this is something fundamentally different from established design principles. Other factions would fire a squad's anti-tank weapon at a vehicle, but the rest of the models in the unit would have to waste their shots for the turn since they didn't have weapons that could hurt a tank. T'au didn't have this problem. They could split their fire between the tank and then shoot a nearby unit of infantry. Or if they had enough firepower to wipe out a unit, they could avoid wasting shots by targeting another unit.

Then we get to Crisis Suits. They could make a special move in the Assault Phase, which comes after the Shooting Phase, which is after the Movement Phase. Crisis Suits basically had jump packs, so they could start the turn behind a wall or thick forest. They could jump over that terrain in their movement phase, pop off a bunch of shots, then use their special move to jump back behind cover. You didn't have time to shoot back and you couldn't catch them because models without jump packs would have to go around the terrain in order to get close.

All of this made for really linear games. The T'au player would either shoot your entire army off the table or you'd manage to get one unit into close-combat which would single-handedly wipe out the entire T'au army. Games against T'au often felt the same with little agency for non-T'au while the T'au basically played solitaire: dice rolling edition.

Since then, changes have been made to the faction to make games more interesting, but the hatred the faction got in those early days has stuck around. They were even more problematic, though, in later editions (especially 5th.) where they would spam undercosted vehicles. This didn't help their reputation.

Anywho, check out this dope ass grimdark concept art by John Blanche: