As a repeat of what people were saying earlier - Guardians/Juggs are kinda weird.
Yes, they are unkillables machines, but they do jack dps. And by that I mean it's so damn low it's ridiculous.
This is a problem because the game relies on dpsing to tank effectively since there are very few "high threat" abilities. Not to mention the complete lack of AoE which is frankly ridiculous.
Meanwhile, I fuck around with a Vanguard, which while has less survivability (I almost never died while leveling with my Guardian while I die often with my Vanguard) is a fucking DPS monster. You can clear a pack of 8 normal mobs with a single attack (mortar volley). It's absolutely RIDICULOUS! Their resource system also works a bit better and encourages you to swap attacks (doing the same one over and over will decrease the 4-ammo count much faster) so you need to be smart about what you're doing, but at least you can open up nicely with a lot of aggro instead of having to build up a resource then becoming resource-starved. If you manage your resource well as a Vanguard, you won't ever become resource-starved, while a Guardian can easily run out of resources if he has to run around too much.
Seriously, it's just... fucked. Their only clear advantage is that they just can't seem to die. They have the best defensive cooldown, the best defense in the game, but they just can't hold aggro it seems (or do any sort of decent damage).