There are ideas for automated bank runs to keep them honest. It really hinges on the key holders being chosen well, though - people of standing in their local communities so they can be held accountable if there is a problem.

When you compare it to a regular custodial arrangement the fedimints seem clearly superior. Say you'd need 7 of 10 corrupt key holders who are ready to collude don't care about the consequences.

Also, any failures (fraud) would be highly localised with this model and obviously the actual supply of bitcoin would remain unaffected.