One problem with legendary saves I've noticed is that it's not "cast three spells and then the fourth spell hits". LR is decided after a save is failed, so if the monster has a halfway decent save against whatever you're using on it (or it has Magic Resistance or Counterspell or is just immune to the spell effect), you're in for a tough time.
Like, let's say a monster has a 50% chance to save against an effect to begin with. So you'd normally have to toss out two spells to expect one to land. With the ability to turn the failed save into a save, it might take 7 attempts!
I know someone will say "well, stock a lot of spells with different saves" but the reality of that is you can only prepare so many spells, and your lower level spell slots quickly become obsolete as the game progresses (and the amount of higher level spell slots is sharply limited). So unless your entire party is in on the "save or suck" game, you're better off just switching to cantrips.
It doesn't help that the threat level of legendary monsters is such that you really need to do something to slow them down, as the longer they live, the lower the chances of survival get, but they're designed to resist things outright.
Honestly, I think the solution here would be for most spells to be "serious effect if saved failed, minor effect if save succeeds" instead of all or nothing (or, as suggested, using LR imposes a penalty of some kind to the enemy for using it).
Or even better, don't use just one monster. Give the boss minions the casters need to lock down so the heavy hitters can just focus on the big guy.