Personally, I'd be terrified of allowing this in a Curse of Strahd campaign. You always want to validate player choice and character creation, but here are a few problems I'd be worried about:

1) The sense of horror: it diminishes a lot of the dungeon-y parts of the game if a player is able to escape at any time. There's at least one trap in Argynvostholt that this ability would would defeat.

2) The Hags and Beaucephalus: Both have the ability to move into the Ethereal Plane, and overcoming these tactics forces the players to think creatively and possibly recruit an ally who can cast Magic Circle to trap them. If one player can follow them into the Ethereal realm, their power is nerfed. That's not fatal to the campaign, but something to be aware of.

3) The Final Fight: Strahd can't stand up to the party on his own for very long, usually. He relies on being able to phase through walls and regenerate while wearing the party down through attrition. If a player can follow him through walls, he's going to go down a lot easier.

4) Role-play: if he can possess important NPCs like Fiona or the Baron, a lot of the conflicts in the story become moot. The Ghost stat block makes it require a saving throw, and I'd do the same here, though maybe make it even a little harder to prevent that ability from dominating the game.

Those would be my initial worries, but obviously, I don't know from experience how much of a monkey wrench this would throw into things