Assassins are pretty good, they have a well rounded skill set and at upper ranks they can mitigate their relatively low HP pool with armor and trance. They can control confrontations with people who can't see them through a mix of the threat of assassinate and just flat out beating them up.


I think one way to level the playing field without really changing their essence would be to give them a choice between concealment and mobility. You could implement it in several ways, adding flat movement lag while hiding, making more commands have a chance to pull you out of hiding and possibly adding less than one round lag to coming out to stop triggers from nullifying the affect, replacing sneak with creep and having wilderness time and civilized time.

All of these things could affect thieves as well if you make it like creep then elf/drow would have a natural advantage (at the cost of their natural disadvantages) in both classes.

If hiding classes cannot keep up with moving targets it forces them into being more ambush related, it gives them a choice, and it reflects reality in the sense that if you are trying to move undetected you can't generally do it at a full out sprint.