We have played the game twice for now. Both times it was a 5 player game.
So far it seems fairly balance, which was one of our main concerns.
Knight, Goblins and Dragon are very interesting characters, whilst the Cave got a completely different game experience. Not better or worse, just different.
On the other hand, the Thief seems a pale game play experience in comparison.
He doesn't have cards to check (sidequest, power, war, secrets, monsters) or confront (events, treasures), or any other components like tiles that require his attention.
He just manages 3 resource token to distribute among atributes, and got a chance to upgrade some options in his board.
This makes far less options that other characters. Even the cave got more options during its turn, with all the Omen Powers, and also got a lot of things to do out-of-turn.
Aye, tactical positioning is interesting for the thief... but this is true for everyone else.
So, we have the impression that Thief is not as interesting as other characters.
But clumsy in comparison
Just an impression after 2 games.
Hope we change our minds on this.
Part of this may be that the Thief was added as a Kickstarter stretch goal. The other four roles existed (and been balanced together) from the beginning, so the Thief role had to be a bit more of an add-on role (to not significantly affect the balance of other roles).
I haven't personally played with the Thief yet, but I see his possibilities mainly being in sabotaging other roles (via Backstab / Pickpocket) and messing with the Knight and Dragon (by requiring Knight to devote more cubes to Perception, and taking treasure that the Dragon needs to awaken). The Cave isn't overly more affected by the Thief, though the extra Vault tiles do add to what needs to be placed.
I can see that he might be a bit more repetitive (as opposed to the other roles' cat-and-mouse-and-dog gameplay), especially if he's just doing his own thing on the other side of the board. I suppose you might be able to house rule that some (or all) of his Treasures must be stolen from the other players or from Vaults instead of being able to take treasures the Cave has placed (maybe reduce number of Treasures needed from 6 to 5 if you do this)?
The thief certainly doesn't have as much to do as the others, but there is still plenty of room for strategy. With the group I played with it was vitally important to have Uncanny Evasion and a high Stealth.
Of course, if you have no one willing to take a less active role then it won't work well for you.