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For such an advanced race, always upgrading their tech, the Tau seem to have very weak stealth suits.
Night fighting rules don't really offer much protection for a Stealth-Suit team (or are my opponents just lucky with maximum range dice rolls)
The tech should allow for one less dice than standard night fighting rules, this would make Stealth suits a very powerful unit and may unbalance the game, but it would also give Tau players a unit that realy excells at the mobile warfare aspect mentioned so often in the fluff text.
I'm probably just talking bull here, I was never really any good at using Stealth Teams. Big guns are more my thing and the Tau have plenty of those.
The Stealth is an insurance policy for the times you cannot get cover or you get caught out. Trust me, if you play the Stealth team as if it did not come equipped with a stealth field (I/E cautiously) you will get much better results with them. Obviously make your opponent take the test, just do not RELY on it.
And as was mentioned recently it's only a light distorting bubble - the suits themselves are cumbersome bulbous exo-armours that get around using jet boots, they arn't stealthy, just hard to see clearly.
Gameplay wise the stealth suits are comparable other 'stealth' rules in the game, given the suit's effective range, where was that table...
Grey Knights, average spot distance 32", weapon range 24"
Stealth suits, average spot distance 21", weapon range 18"
Harlequins, average spot distance 14", weapon range 12"
It's more of an 'anti-gang-up' thing - makes it more difficult for things to shoot at you when you can't shoot back.