I have 14 Angelicus loaches and 3 Clown loaches in a tank. I had a serious MTS problem in that tank until last tax season when I didn't feed them for a week or two. I found 100+ empty shells in the tank. I had thought the MTS were completely out of control when I went to do maintenance on the tank after tax season, but there were hardly any left. I tried establishing a population in the tank again, but the snails just do not live long enough to reproduce.
From what I have heard, the best snail eating loach is the tiger loach, but the yoyos do a good job too. Loaches will eat assassin snails and mystery snails. The reason your yoyo probably has not touched your mystery snail is because the mystery snail has a trap door and isn't as easy to get to as the ramshorn snails that do not have a trap door. Plus, it has probably been well fed on the ramshorn snails. Once all the snails are gone in the tank, you can bet that the mystery snail will be pestered to death, especially if you have a group of loaches in the tank.
The mystery snail and assassin snails will make an expensive snack for the loaches.