To add to what is already said - which is all spot on - there's one more nice little trick that you can pull with Defenders: combine psy power "Protect" with stratagem "Celestial Shield".
Once you drop your 20-strong unit of Defenders (ideally with a couple of platforms) from the Webway into the enemy position, you can cast Protect on them. Next turn, when your opponent decides to focus them, you declare Celestial Shield - and all of a sudden your Defenders are protected with 3++ Invulnerable save (while platforms also have 2+ armour). That can be outstandingly strong.
I agree with most previous comments in that Guardians can (and imho should) be used as "heavy-hitters", in which case they are better fielded in 20-strong units - and in that case they are better deployed via Webway. Under the current rules, they'll only arrive on turn 2, which makes them a sort of "2nd wave attack" unit. Note that due to their short range they would almost never be able to attack anything on turn 1 anyway, so Webway deployment really has no downsides.