I've only faced them once, in a custom battle. I was playing Getai vs the Aedui, who had 1 unit of Gaesatae. First, I thought about using a Sarmatian merc unit to lure them to a chase, but that turned out impossible because they were surrounded by so much other gallic infantry that I could never get cavalry anywhere near them. What I did was order all my archers to focus fire on the Gaesatae. It worked very well, for they spent quite some time lugging javelins before finally closing to melee with my line. By the time they did so, they had been shot so full of arrows that they routed very soon after attacking my phalanx. They didn't seem overpowered to me, and like I said I never even had the chance to use cavalry against them.