After dying to rotten damage due to a legendary contraption going off on floor 41, when I'd been easily clearing floors before made me think of how to improve them.
My thought is one of two avenues:
One, make them go off at end of character turn, rather than end of monster. Makes them more powerful, yes, but also makes it feel like something you do. This means kickbacks from imps and rotten are calculated risks, rather than a random threat based off random targeting.
Two, make contraptions actually ignore mechanics. I personally like this idea more, as it gives them an interesting niche. It doesn't make a lot of sense to get hit by rotten because a machine threw out a flamethrower, after all. But it would also give a good niche against imps and scorpions. You'd want more up, because there'd be less threat to having them.
As it is, I find Tinker is played best without damaging contraptions at all. Maybe a stun one, and some utility ones, but contraptions are its weakest point, for sure.
Another small aside is that if contraptions kill something while Team Effort is up, then you gain block at the end of monster turn, and then lose it when it becomes your turn. I feel like it should stay.