There isn't a problem by itself. Combine all the pieces together and it neuters casters. A 50% chance to save against spell durations that allow a save on a round to round basis or a save against the damage for a 6th level resource you get two of per day is a big deal. It's not like 3E where you have four or five six level slots, a bunch of magic items, and scrolls for additional magic. It's two 6th level slots a day that only have a roughly 50% chance to work for one round. If they happen to miss that save that works for 1 round, they can use legendary resistance.
The average legendary creature has 3 uses of legendary resistance. The average high level caster has a total of 6 spells level 6 to 9. 50% chance save chance from base saves combined with three chances to automatically save, you're looking at effectively being able to completely resist a high level caster's best spells 100% of the time on average. Add in magic resistance, you have a worse chance. Throw in the concentration mechanic, you have fewer spells on your list you can use. It all adds up to very neutered capabilities against legendary creatures.
It's not a problem from a party perspective. It sure is boring from a player perspective for caster.
As I stated, you're better off playing an Eldritch Knight archer or rogue archer with some spell casting to have the ability to be effective in nearly every scenario within the game.