MEL cases must be "fully certifiable" and comply with everything in part 25. It's effectively a new version of the same aircraft that has a difference in equipment, and must meet all the rules.
Failures don't have to meet the certification rules, except in so far as they have to meet the ability to continue flight and safely land (or similar wording in different regulations)
Thus limitations for failures are rather different than MEL.
For example, you'd never get MEL for OEI dispatch
Waaay too many rules not met. But you still have a procedure to fly if the engine fails.
edit: to say that the MEL'ed aircraft obviously has lost redundancy etc, and may not meet the safety rules for the next failure if left on the MEL for the life of the aircraft; the MEL time limits are determined to bring that consideration back into full compliance