|Stupid blue fishgoatmen.|
In short, I think Tau has all their weaknesses covered. Being able to take allies is one thing, something that all armies can look forward to for the most part, but there's something else much more worrisome. When you look at the Tau from an external balance point of view, I think they're the kings of the battlefield. A lot of the mechanics that they bring to the board outright counters others. The best example of this is how they can outright remove cover from something. In the last book, this wasn't a very common thing and you had to pay to play with this luxury. Not so much in the new book: you have very cheap ways to get SMS, Pathfinders rock, Marker Drones are everywhere, and there's even viable Skyrays. I don't think any ability should outright remove another ability; it's just bad game design and completely negates a defining feature of the game.
I honestly don't think the balance of the Tau codex is in the right spot. They seem to annihilate most of their counters and make up for their weakness of low I, WS and poor CC with outright impossible firepower and extreme overkill. Not to mention most of these scenarios ignores cover, is twin-linked, has volumes upon volumes of high S attacks, and is just capable of destroying most answers forced to meet them. There's Skyfire, Intercept, but most of the time you don't even need protection vs. air because you just twin-linked and/or marker it to death. Broadsides stand out as the most problematic, the Marker Drone Crisis Suit Leader is another, the +1ROF Ethereal with Fire Warriors on OW duty, the Riptide being god damn amazing, and someone tell me why would you take anything else other than SMS?
1/5 for External Balance. Get some real playtesters GW.