Just read some of the legendary spells and they seem very weak to me compared to other lower level spells.
You have the spell Disintegrate 6th level that can cause 40d6 damage at level 20 and most legendary spells don't come close to that damage.
For example, Disintegrating Blast 12th level spell will do 30d6, doesn't matter the level of the caster.
So yeah the radius of the blast is bigger and it can effect more then 1 creature, but still, nothing makes it legendary with this kind of damage as a level 12 spell IMO.
Other spells follow the same road.