Mayhem Devil and deathtouch
There is a problem with having two Mayhem Devil cards on the battlefield, under your control. One of the Mayhem Devil cards has deathtouch from Call of the Death-Dweller. The other Mayhem Devil does not have deathtouch. I sacrifice the one without deathtouch using Phyrexian Tower. Problem: I get no indication of which 1 damage ability on the stack is from the Mayhem Devil with deathtouch and which is from the Mayhem Devil without deathtouch. It should be more obvious when you mouse-over the triggered abilities on the stack.
People will be losing games because you cannot tell which Mayhem Devil is doing what. It should be explicitly spelled out in the triggered ability on the stack.
Your counter-argument before you even make it: "there are lines from the Mayhem Devil onto the stack that tell you where the source is."
Well, no. Those are ambiguous. Explicitly spell it out.
If multiple Mayhem Devils get sacrificed at the same time, one with deathtouch and one without deathtouch, you aren't going to be able to tell what is going on. Please fix this.