I've played against them 3 times now.
The list I played against had 2 Flyrants allied in for anti-air/ general killyness.
The hard thing playing against the Genestealer cult is that there really isn't any good way to game plan against them since they can come in basically wherever they want.
Reserving against them is very risky with the -1 to reserves.
One of the things I took away from my game is to make sure and fully wipe squads, don't just leave one or two models since they can go back to reserves and come back out next turn with +
D6 models.
Depending on game type it may be a good army to castle up against and plan on a good counter assault.
You can check out two of my games here if it helps:
https://www.youtube.com/channel/UCRNXFaHZqv45sl_VDySwCuQ