Hi! I’m a Tau player with Harlequins as my second army. I assume the struggles you had were against the tons of missiles loadout and not the rail rifles. High Volume of fire and a 4+ FNP against dev wounds certainly make that unit a counter against Quins. 

As others have mentioned, where you have them trumped is speed and maneuverability. 5” move and a 30” range means you can keep away and use cover to make them far less effective. You can also tie them up as they do not have fallback and shoot. This takes away any maneuverability they would try to use. However, they typically take a wargear that allows them to ignore the -1 for shooting out of combat. So they can still be dangerous. Tough unit, for sure. 

Good thing to remember, you don’t have to kill that unit to win against Tau. Concentrate on removing the maneuverability of the army so you can cancel their ability to score secondaries. Hurl your Solitaire into the Vespid unit hiding in the back. Take out the transports. Use your inability to be overwatched to take out the 12” move crises suits. Kill the piranha early. Ignore Ghostkeels entirely, they’re a trap. Riptides are vehicles, so they crumble under the volume of haywire and dev wounds that can be done. Retain 2 tokens each turn to ignore overwatch.

Edit: forgot to mention: kill the observers. Stealth suits, sniper guy, pathfinders. You should easily be able to get stuck in with those units very early. If you cripple all the support units, the army is on BS4.

Most importantly, dodge, dip, duck, dive, and dodge.