Is Saboteur biased towards the miners?
Saboteur 1 does seem to be biased towards the "good dwarves" if you don't play it in really cutthroat fashion.  If the good dwarves just cooperate and go for the gold, they will win most of the time.

However, if the good dwarves break each OTHER in order to keep competitors from getting gold, it gives the Saboteurs a much better chance, obviously.

I think Saboteur was designed with an expectation that good dwarves would be more cutthroat and selfish than a lot of groups play them instinctually.

That said, I recommend you pick up Saboteur 2, which adds (and replaces) some cards and roles.  It's much more well-balanced and more consistently fun.