There are a lot of people (including me) that complain that in WvW thief’s are to ‘strong’. With strong I mainly mean that the combination of very fast and invisible makes them very hard to kill, besides that they can do a lot of damage however there are things you can do against that.
But in addition there is also a bug that makes them even harder to kill for a lot of professions / builds.
The problem is that invisibility also makes thief’s invulnerable for multiple attacks.
An example is all ranges attacks that bow’s have, like Point Blank Shot and Rapid Fire. The AoE work but the normal range attack wont work. Another example is the downed skill Thunderclap.
In my example I pick Point Blank Shot and Thunderclap because I use them to interrupt people who try to finish somebody off.
There are some skills that other professions have that have similar behavior like the skill that prevents you from getting dazed or crippled and a skill that protects you from range attacks. But this is something different. The problems / bug seems to be in the fact that there is no target and the system/game can’t handle this.
The whole idea behind invisibility is that it is harder to damage them or do something against there attacks because they are invisible, but this problem now even makes them invulnerable (for some attacks) even if you use the correct skill on the correct moment or you do shoot correct at the invisible thief.
If you do know of this bug you might want to switch to melee but still it should not be the case. Most thief’s are also abuse this bug for example when they finish somebody. (not saying they are all aware of it. They will just notice that they almost never get interrupted when they finish somebody in stealth)
I did find out this bug because when a thief is finishing somebody you pretty much know where he is however the Point Blank Shot never ever works and Thunderclap also never works when a thief tried to finish me.
Personally I also think that the combination speed and invisibility makes them to good at getting away. It makes it to easy for a thief to try and back-stab and if they see it does not work (they lose the fight) they can use a combination of speed and invisibility to get out of the fight. Something that does not really seems fair as you should have the possibility to attack the person that attacks you. However adding the partially invulnerability to it really makes it to much. (for some builds / professions)
Some professions / builds will not have this problems others do. See for example rangers, they usually use ranged what becomes unusable, they have the thunderclap that becomes unusable and there pet is also unusable (that the pet is not useful for attacking a invisible thief is not a bug, but it adds up to the total). I think professions like engineer will have similar problems but a warrior will most likely do not have any extra problems because of this bug but only haves to deal with the invisibility and speed like what he is supposed to have to deal with when fighting a thief.