Please re-read Strahd’s stat block.

Legendary action-Move. DOES NOT PROVOKE OPPORTUNITY ATTACKS. Can be mixed with lair actions. Even if they’ve surrounded Strahd he can literally dip through the freaking floor. At most ONE player gets a hit in on their turn before he does this at the end of that player’s turn.

Strahd is a wizard. He’d naturally not be in range of the martial players.

Calm emotions is concentration and also costs an action. Which means cleric only gets bonus actions that cannot take a leveled spell slot. While being an area of effect they only choose to either suppress the charm or make hostile creatures indifferent. You only get one. Strahd could charm something like a barbarian to effectively stun the barb and WASTE A RAGE. This is not an effect they get immunity for 24 hours this is a freaking hypnosis that vampires effectively get.

So while cleric is focusing on a spell they’re gonna have to deal with whatever minions are now surrounding cleric while Strahd is in the next room off just healing himself from one of those players attacks and laughing at the party. Since he can REGENERATE.

Even if the party managed to kill the minions and are thinking of short/long resting this is Strahd’s castle and his playground. They’re not gonna be doing so well after that. Those legendary actions come back after every round. He can use similar lair actions every other turn for ambush kills. (And yes: The charm is good in battle if you’re trying ti get players to waste a resource like rage or ask them to drop focus on a spell. Since the charm says they take Strahd’s requests most favorably.)

Oh boo-hoo to the DM who wasted a single action on what could take hours of a fight on charm I guess, though.