Man, there's a giant 'Instant Death-dealing' hole in the Tau armory, period. Yes, railguns, but most other armies can spam long-ranged S8 weapons (krak missiles, psybolt-autocannons, lances, lascannon, Battle Cannon, etc.) in higher numbers, on a wider amount of platforms, many of which are more survivable thanks to being in units with relatively cheap models or transports. It's a particularly brutal comparison given that XV8s are expensive two-wound models who are easily one-shotted, while ten guardians with an EML, or a half-Tactical with a lascannon, or a full Strike Squad with two psybolt Psycannons in a Rhino are as tough to remove as they are deadly to the vast majority of T4-or-less models in the game.