The final exam is no easier than the midterm.
If we substitute not easier for no easier here, will it change its meaning?
Slightly. 'Not easier' means just what it says. 'No easier' mean the same, but with the added implication that someone (either the speaker or the addressee) hoped or expected it would be:
Speaker's hope disappointed: I waited until after the rule change, hoping for a cushy exam, but it was no easier.
Addressee's expectation not met: I don't know why you don't do it now. It'll be no easier if you do it next week.