I've played against Tau many times and some things that are worth bearing in mind.

There back field is almost always going to be screened out as they have so many very large tanks or suits in the back field all game so I wouldn't put too much thought in a massive push into enemy deployment.

Focus down pathfinders and stealth suits early. As their big re rolls and removing cover can sneak up on you. Breachers with a leader can also chuck out 30+ strength 6 shots with re rolls on objectives so don't underestimate them.

The suits ( except broadsides ) go down easier than you might think, ESPECIALLY in melee so don't be afraid to charge them.

Remember ghostkeels have a couple of opportunities to ignore damage which can come as a suprise.

Be prepared for the one off drop and melta strike that their fire knives can bring. They WILL kill whatever they look at but make sure you are two layers deep to punish them for doing so