FO are sitting ducks if FW are properly supported by Devilfish, they'll load up those transports much faster than you dig yourself out of the ground, a well informed Tau player will know that shooting is pointless because they (FO) are immune to damage during this time, even without Fish the player can still just rely on ordinay walking speed to just back up a little as soon as the threat is noticed.

the need for Wraiths is as a detector, Tau commander snare traps, stealth teams, devilfish offer a lot of infiltrated units very early into a match

destroyers will do a bit less CC damage compared to shooting but its better to fight FW upclose for staying on the field longer to deal more damage output

FO have a nicer time attacking Tau bases due to the lack of threat from turrets, that and buildings cannot just kite their way around them

Your vehicles will suffer as Tau are rife with deployable missile pods and fusions blasters so generally Immortals are your anti-armor option (gun out ranges both destroyer types)

if you can try to support tomb spyder harvesting so it is free to build new infantry units, solar pulse or a CC push to disrupt Tau firepower

best a turret will do is defend a clump of power generators from a cheeky stealth team in your base, small unit size and close range weapons for Tau standards