I agree with more mcs on the field it favors sniper heavy armies. Gosh the Neff on pathfinders for Eldar. The snipers for tau do take up a heavy slot, but that is what O'Shiva as allies come in
I'll double check their Sr, but I thought it only applied to them and maybe any IC that joins them.
The problem with the cost effectiveness is it limits how many you can field. 2-3 squads to me would seem viable to deal with the abundance of MCs on the board. The problem with one squad is anyone playing with MCs will try to take them out, gravity guns are a pain since you can spread them out throughout an army.
My second gripe about them is they are really effective against one target and you have to declare what that target is. If it was hidden than they would be much better. Since as an opponent I will know they would be strong against me I would stay away from them.
However they would be rather good if you have them back field with a crypt with a veil of darkness. That way you cannot deep strike and possibly rapid fire your desired target. Depending on how the creek is geared they can do some damage that way.