Gringotts is a very good themed experience. As a ride though, it does not deliver on the concept it promises. You assume a wild card ride and instead mostly sit and watch action play out. Even more weird is that the ride system and ride vehicles are designed to perfectly emulate what you see in the films. For that reason, it's not super re-rideable. I will also say it goes a little too hard trying to place you in one very specific moment from the books/films.
Mummy is a much more fun and re-rideable ride, but from the standpoint of complete immersive theming, it's actually a bit of a mixed bag. The story is ridiculous, the showpieces in the coaster part are just UV paintings in a warehouse. BUT - the coaster part is great, the dark ride portion is great, and the pyro effects are also great. Honestly though the weirdness and slight cheesiness of it just adds to the experience. It's honestly the perfect old school theme park attraction that doesn't take itself too seriously.