Mayo works pretty well in most savory sandwiches that are on white bread. I think that mustard works better on rye bread or with corned beef or Swiss cheese. Ham and Swiss on rye really needs mustard. On the other hand, ham and American cheese on white bread can take mustard, mayo, or both. Hamburgers can also go either way.
When my daughter was a toddler, she went through a phase of wanting to eat either PB&J or McDonald’s for Every. Single. Meal. I was willing to let her have ONE PB&J meal per day, and since she didn’t like cereal or eggs, she generally got it for breakfast, with a glass of milk. And since I was making a sandwich anyway, I got into the habit of making one for myself as well. I still eat them from time to time, though nowadays I generally skip the jelly. Many adults regard a PB&J as a comfort food, and that’s one reason why we have such strong opinions as to the type of peanut butter and spread and bread. We tend to want whichever kind we had or wanted to have when we were young.