On the official Dark Messiah forum, megazero reports an issue that has been debated long before the start of the open beta: the Priestess class is overpowered in multiplayer. A screenshot is included to show what kind of score (61-8) a good player can achieve using this class. Further down the page, Oakwarrior points out that the class isn't doing what it is supposed to do in the first place (i.e. heal other players), and describes how hard it is to kill a priestess:

"You can't chase them because they'll slow you and kill you, you can't backstab them because they'll most probably sit on their own brambles and they can see assassins (fortunately most of the time they're so busy killing everyone else on your team so you may get in a backstab IF IT WORKS) and bombs are a pain in the ass to use against them, you can't kill them with spells because they'll just heal themselves and run to a safe place while throwing brambles at you (...), you can't kill them with an archer because they'll run throw brambles at you and you can't do anything because your rate of fire is too slow for THEM."

If you have been playing the beta, you know the feeling. Unfortunately, time is running out as the game is scheduled for a late October release and will need to go gold eventually. At this point, I'm not sure Arkane and Kuju want to make any major change to the classes, given the amount of time left for testing. You can get a summary of the Priestess skills here -- it will be updated based on the open beta soon.