During the course of playing any miniature war-game (or most any type of game) you'll come across a model, card etc. that "breaks" the game. This doesn't necessarily mean the piece in question is overpowered or unbeatable; rather it changes the nature of the game.
What this leads me to is my least favorite faction to play against in Malifaux. The Neverborn are the bane of my existence. The general consensus amongst players I've spoken with is that Neverborn are "unbeatable" or otherwise "unfun" to play against. While opinion cannot be legitimately classified as right or wrong, I do tend to severely dislike playing against them. I could be playing against my favorite opponent, but as soon as those purple cards hit the table I lose a bit of my enthusiasm.
The question then comes up as to why this is. The models are relatively "balanced" and once you figure out how to deal with them their weaknesses become obvious (still working on this one...). I think what it boils down to is every one of the Neverborn masters somehow ignores or changes a core mechanic of the game. Which in turn means that when playing against them you have to change your thinking and read and re-read their abilities. That I think is where the "unfun" issue comes up. When I have to try to remember every weird interaction possible, and my opponent has to do the same thing, games tend to drag on and get bogged down with looking up rules and interactions.
In my opinion the Dreamer and Pandora are the worst offenders. Both significantly change the way the game is played and involve more "single-player" aspects than other masters. Pandora is annoying because her ability revolves around Wp tests, in order for her to work she removes the immunity that some models have to Wp duels (and pay for). The Dreamer just makes my head hurt, mostly because his crew involves much more thought in dealing with and playing then any other crew in the game.
Regardless of what Neverborn Master you face you will have to deal with the fact that some aspect of the game becomes irrelevant.
Lilith – Ignores Terrain, Creates New Terrain, and can swap any models on the board
Pandora – Removes immunity to Wp, causes wounds for tests that you may or may not usually have to take, jumps around the board.
Zoraida – Takes control of you models, can hurt you indirectly, can ignore terrain
The Dreamer – ignores initial deployment (for the most part), can pop models up where ever, can't be targeted (under specific circumstances) and a few other weird thing I'm forgetting at the moment
So are these models "broken," probably not. There's ways to deal with them and ultimately playing to your strategy or schemes and avoiding them can win you games. Are they "unfun," that depends on your perspective. For me I have a hard time getting my head around them and remembering all the odd interactions so they border somewhere between annoying and passing on a game (depending on my mood). That's not to say I haven't had really fun games playing against Neverborn players, but on the same level I've also had plenty of unenjoyable games against really great opponents that are only unenjoyable because of the crew I'm facing.
I hope this doesn't come off as whining or complaining. There's nothing wrong with any of the models described above. The issue for me is really just finding the way to deal with them while still having fun.