For those not aware:
One of the major reasons why Thieves currently appear so broken is due to a technical shortcut/cost-cutting measure made by ArenaNet to reduce server load and “resolve” culling. The game currently polls for character models every 2 seconds. This means that if a Thief happens to drop stealth at the right time (purely luck), he will remain invisible for up to 2 addtional seconds.
This needs to get fixed so that Thieves are not nerfed due to bugs. We can evaluate the relative strength of the class once they appear out of stealth instantly, as designed. Otherwise, we will require nerfs to balance the class while ArenaNet experiments with fixes.