Sad that it is stated as the noobie-to-go class :< ..
You are right about the forums going negative on us; We lack damage and threat, especially AoE wise - But you can make it work if you know what you are doing.
Yes you are viable as Hardmode and operations tank! (though only if you learn to properly manage threat and output)
As guardian you are more avoidance based (defense rating) than your counter parts trooper and Jedi Shadow - However, you have the best defensive cooldown available at the moment.
The most important thing is to manage your resources correctly, and make sure to CC (crowd-control) and mark your kill-targets.
Furthermore, is the FP-Hardmodes currently harder to tank, than the actual operations (with Soa as an exception); so make sure to go Hardmodes with friends, to diminish the lack of AE threat and damage we currently suffer - Since they will know not to attack a random target if they want to live :)