I mean personally all sub places are mediocre. It's just bread, deli meat, cheese, and mayo or whatever condiment you put. You can easily make the exact same thing at home.
Well, the quality of the meat is major. There is an ocean of difference between the middling priced meat you can get from Sysco (or random restaurant supplier) and Boar's Head. Next is how fresh a place keeps their produce. And finally the bread - do they bake it inhouse? Is it any good? If they don't bake their own, do they at least get a few deliveries a day?
For a fast casual place Jimmy John's has better meat than, say, Subway, the bread is decent, and they refresh the produce frequently.
But I sort of agree. I prefer to make my own subs using a bun fresh from the bakery, Boar's Head meat and cheese, and produce I just sliced myself.