No doubt that a lot of effort went to create this campaign and the amount of content is amazing, but as a whole it's being seriously held back by both Thief 3 shortcomings and some design decisions too.
The OM modifications are quite creative, but from the beginning, gameplay relies on running, jumping and mantling, which is constantly glitching due to engine's animation and physics problems.
In terms of difficulty, I have an impression that there are always numerous guards everywhere, with very tight patrol routes, and switching to easiest difficulty does little to change their count or abilities. In "Streets of Rage", I got a guard spotting me in complete darkness, for example. Also, some missions have strict "no KO" objectives, which may not appeal to all players. If you're in a more hardcore crowd, this is probably a campaign for you.
All in all, I think it's a very mixed bag, but since there are 12 (!) missions in total, there's a chance you find at least one you like.