There are armies I like, fluff-wise, and others I hate, the same. Personally, I do like the Tau, once they became a bit less "the Good Guys Amongst the Sea of Evil", and became more "oily, seditious aliens who will trick you into slavery, as if it was your own idea, and sterilize you, while you work." Some of the "Asian" themes, a lot of which I didn't see until the Dawn of War series did it, gives them a Neimodian feel that makes them less trustworthy, to me, and thus more likeable. I apologize if that offended anyone, but it's sort of "how I feel"; maybe we can blame George Lucas. I liked the Necrons more before because sometimes the "personality" attached to factions of unchanging, Human-hating hordes seems pointless. How much soul (hah!) do you need to impacably kill every other creature, without any regard for anything else? Also, they were less "the Tomb Kings from Fantasy in 40K" before they basically became just that, with fearfulness, cowardice, backstabbing, plotting, deal-making, diplomacy of sorts, and a lot of ancient, broken-down tech. I won't even go into them making team ups with Space Marines. Also, they whipped their own gods, but haven't done much since; how tough must those gods have been?
Conversely, I differ from a lot of 40K fans in that I HATE Orks. I honestly don't like anything about them, and don't find them funny, amusing, or what have you. From their cockney accents to their "we're so dumb, but numerous, the very galaxy does what we want" mentality (red paint goes faster), I just loathe them, and am only glad that they almost never hit anything with ranged attacks. Not a big fan of Dark Eldar, either, fluff-wise. Being d***s just for the fun of being d***s wears old with me. Slaanesh I hate from a 4th ed mechanics angle, mostly, with their Mark making Noise Marines unfun to play against. And things could go on.
For tau ships, the only problem I'd have with them is that they are Tau. In that, I mean players aren't really likely to have them, couldn't upkeep them (who'd know how?), and the Tau are SUPPOSED to have no real presence in the Expanse. Granted, your game isn't my game, so I'll not tell you what's right, and you COULD be happily doing one of my future plans; Rogue Trader in the Reach, but... As for ship strength, I think they maybe SHOULD have stronger guns. Rail tech is awesome, and the Tau can actually make stuff, which the Imperium can't. The Tau should be relatively superior in space; they can innovate, make smaller, more compact stuff, mass produce, and such. Their weakness is distance; they don't have Navigators, and their ships can only skip across the warp threshold, anyway, so their uneventful voyages are slow, and they are limited in where they can go. Once they enter an area though, they can often hold it for a good, long while, ergo the Jericho Reach. The only thing likely to shove them out of there is the Tyranid Hive Fleet, as Dagon is still, IMO, likely to eat all three other competitors, eventually, and even if they do lose, worlds they touched will just sprout more organisms later, and on it will continue.