Has anyone substituted raw mustard greens, collards, or other stern leaves for kale in salads? How did it work?

Frankly, neither collards nor mustards are tender enough to be eaten raw. Plus, neither will reach their full flavor potential unless they are simmered to tender, to break down their tough cell walls. A professional tip: in restaurant kitchens, even very tender & young kale leaves are generally torn very small and massaged first, and rested, before they can be served raw. But really, that category of greens taste MUCH better cooked.

Collards and mustard greens can be really tough as well as bitter. If you have very fresh and young collards and mustard, you can add them to salads. The young plants are often tender and sweeter.

I eat all of those greens you mentioned - each has a different level of bitterness and "green" taste to them. Kale is the least bitter and most mild flavor, that is probably why it is the most popular. If you don't mind the bitterness of collard greens and especially mustard greens, your salad will be fine, I personally like any of the greens with a bit of garlic (and sometimes bacon), but I would say to try each out to determine which you like in your salads and which you think are too overpowering for your taste.