I play mirage in pvp and yes, I find it very frustrating to play against the class as a new or casual player. (i hated playing against mesmer like 1 year ago)
Mesmers can stealth, teleport away, dodge, evade, invul. Also it can put lots of conditions on you that damages you heavily if you try to play the game (read walk or use skills - torment/confusion).
The UI doesn't help a tiny bit since the conditions are hidden in a tiny area down below... While you need to pay attention to character animations, enemy boons, team fight status, minimap, cooldowns, etc... Everything is distant from eachother and there's no way to reorganize the UI to your liking. So, you might not even know that you have high amounts of Confusion or Torment before the big damage hits you.
Besides all this, mesmer also creates clones of himself and can break targeting with a few skills. Which makes it almost impossible for newer players to identify the real mesmer which leads to frustration.
The class is really cool, original concept but its too flawed to balance it out sadly IMO :(