Fedimints are based on HoneyBadgerBFT protocol which can survive up to k traitors among 3k+1 nodes. For example, in a 7 node mint at least 3 nodes must turn evil in order to pull the rug.

A large mint is not a large organization in the same sense as a centralized exchange. A large organization may run just one node of a large mint, more than one would be too dangerous.

I'd take such a setup by organizations that have reputation to lose over a centralized exchange any day.