Cool concept, badly developed - T'au
Bad concept, unsupported by lore - IK army
Fine army, broken rules - SM
As someone who wants to play Tau in 40k, I'm in agreement. I like Space Marines, don't get me wrong. But the Tau honestly always seemed like the coolest faction, even after GW tried to placate the Grimdark fans by turning the Ethereals from wise rulers to "vaguely untrustworthy, possibly evil mind-controllers who may or may not be just as authoritarian and ruthless as the Imperium, just in a different way" kind of characters. But the concept of a small, idealistic, high-tech race that genuinely wanted to work other races and formed an almost Star Trek-esque coalition really stood out as a contrast to the xenophobic, backward, dystopian vibe the Imperium gives off.
But I haven't gotten into playing them at all yet because frankly, their rules suck. They essentially only get to play the game in one phase, the shooting phase. No melee, no psychic powers, a playstyle that encourages them to stand still and shoot when they're supposed to be an extremely mobile army in the lore, but except in a few editions not nearly enough firepower to actually make the shooting phase count enough to make up for not being able to do anything else.
The fact that GW seems determined to keep pushing the "Tau can't melee" idea actually makes them probably the hardest faction to balance properly. They've always run into the problem where they're either so overpowered in the shooting phase that other armies can't do a thing against them because they just get shot to pieces before making it into combat, or their shooting isn't enough to stop enemies from getting into combat with them and they almost automatically lose once the enemy closes on them.
The entire 9th Edition has been a significant nerf to the faction so far. They're going to need a complete re-work in their new codex if they're going to be compete. Smaller table sizes mean you can no longer keep your units away from melee for any length of time. Crisis Suits can no longer jump out of combat with jetpacks. In fact, the jetpack keyword basically does nothing at the moment. Buffs to blast weapons now discourage taking larger units of troops, which nerfs Fire Warriors hard because they want large units putting out lots of shots. Shield Drones, which have always been annoying to the opponent but were also the only way to keep anything alive for more than 1 turn have been nerfed.
Not to mention the constant buffs to Space Marines, the most prevalent army in the game, have made it almost impossible to play against them. Only the Necrons, who thankfully got a major rework, are able to put up a decent fight against them right now. Necrons also hard-counter Tau at the moment due to the C'tan shards being massively OP, especially the one that can only lose three wounds per phase, and heals 1 a turn. The Tau literally cannot kill it until turn 4 at the minimum.
Honestly, I think it comes down to GW unintentionally allowing all the "Tau can't melee" and "weeaboo space communist" memes about them influence them into thinking that the majority of players really do hate them. They only want to focus on stuff they think will sell well, and of course that's why all the constant support for Space Marines. But if it's true and most of the fanbase dislikes the Tau, why bother making them good on the tabletop? Which also brings up the point that in every edition where they've been good, people have complained about them non-stop, mostly because of the poor balance issues mentioned above. If they were better balanced and able to interact with all phases in the game, they wouldn't be such a "controversial" faction.
If I ever do start playing 40k, I'll probably get at least enough of them to play 1k games.