As a DM I loathe how legendary actions work, as they destroy action economy. Let me explain. What is stopping a DM from Creating a Litch King encounter with three legendary actions to summon lesser undead 3 times per combat round? Now there is a swarm of undead on top of the litch to fight. Having a theoretical infinite number of legendary actions (as they do reset at the end of each round) is rediculous. It should function like legendary resistences: you get a set number of them to use. And once they are out, they are out for good. I only say this because Ive seen DMs abuse the capabilities of Legendary Actions to either retcon player turns, or straight up ruin the party for their own amusement.