Well, a great many DMs - myself included - find that he dies in a round or two of open fighting, so he clearly can't be that strong. There just isn't time to make use of those abilities if he - as happened in my game - only gets a single action before dying. This was while standing in a large chamber with four Vampire Spawns and a heavily-wounded Rahadin backing him up, and friendly NPCs in the way of AoE blasts.
The line one hundred adventurers is about the graveyard in Barovia Village.
I think you were likely not playing him up to his full capability. He should never only get one action before dying...he can use Legendary Actions to get up to 4 turns per round! If he's that badly hurt that quickly, he should retreat rather than remain and die. His abilities make it pretty easy for him to get out of a battle; he's incredibly mobile.
As [MENTION=6787650]Hemlock[/MENTION] and [MENTION=6785438]Warmaster Horus[/MENTION] point out, he's very dangerous. I had my group of level 9 PCs just about dead in their first major fight with him in his treasure vault (his location based on the tarokka reading) and then a second battle at his crypt, where everyone was basically on their last legs. In the first battle, he often used his lair action to be able to move through walls and floors to get around the battlefield. At one point, several of the heroes had moved into the vault....he fled using his Legendary Action to move without provoking, and then used his Lair Action to shut the door behind him. That left a few of the stronger martial types uselessly stuck within the vault while the bard and wizard were stuck outside with him.
Those are some seemingly minor actions that can have a huge impact on play. But they have to be put to use to be effective.
I think that this is a big part of why we see such varying opinions on this stuff....there are such varying degrees of tactical play. Some DMs seem to want their monsters to just be able to go toe to toe, turn for turn, with the PCs, trading punches until one side or the other drops. Other DMs have the monsters remain mobile, and thoughtful, and try to avoid taking damage while trying to maximize their damage output. Most probably do a bit of both. None of it is wrong....but all of it will affect how monsters perform.
I think that Legendary Actions and Legendary Resistance should be given to ANY monster that the DM wants to be a solo battle for the PCs. It makes the monster more on par with a group of opponents in the sense of actions per turn. I think this is what the designers intended, but I don't think that they made it so blatantly obvious that everyone realizes it.
I look at those as the equivalent of Feats or Multi-Classing for monsters.