A squad of 20 guardians coming out of the webway will probably wipe a 5-man tac squad with minimal difficulty, and might even be able to kill 2 if it has a couple Shuriken Cannons. Depends a lot on those rends, of course. I think the goal with something like this is to shoot at chaff units like cultists if you can, and present a big threat that can sit on an objective, and leave a point open for Celestial Shield so they're hard to kill.
I think if you want deep striking anti-infantry firepower, Eldar have other options like Warp Spiders and Swooping Hawks, and Swooping Hawks are in some ways better, 20 guardians with Shuriken Cannons costs about the same as 15 swooping hawks, who put out a blistering 60 shots (plus grenades etc) that bring them pretty close the guardians at shooting MEQs out of cover, much better than guardians at shooting guardsmen and such. of course, Guardians' rending shuriken weaponry makes them the winner against MEQs in cover, and they still outperform when shooting MEQs out of cover by a reasonable margin.