Thinking about the problem of integrating the new races into a Ravenloft campaign, I think it could boil down to this: It's not what you are, it's what you do.
Remember, in the old Ravenloft campaigns, there were the Caliban, hulking misfigures brutes born under a curse or a bad star. Those weren't generally burned at the stake at first sight, either. One of the 3rd Ed Ravenloft books includes a caliban travelling Ravenloft (from Rokushima, I think) that hs that head of a tiger. The leap from there to a dragonborn is not all that great. With Eladrin, I see almost no problem at all, seeing that they look like elves, mostly. Tieflings are problematic, but can probably be thrown under the Caliban excuse, too.
The thing is, except for domains with an active inquisition like Tepest, villagers will NOT burn you at first sight if you are malformed. They will distrust you, keep their children from you, order you to go away and so on. It may be hard work and take some heroics to just get them to let you stay in the shack at the edge of town. But my understanding is, mobs form because things happen, not because you are malformed. An Eladrin will not be killed as long as he does not show off his teleportation. A Dragonborn will create a fearful mob with breasthing fire, but not by having scales.
One reason for that is that while it is true that in Ravenloft, evil tends to grow unless kept in check, it is even more true that he who asks for it gets it. There is no thing more dangerous than standing in for what you believe here. Curses are so frequent that every peasant will think twice to kill a gypsy or caliban, lest their curse be transferred to him. Why is so much evil afoot in Ravenloft? Because it is safer for people to try to ignore evil than to make a stand. Burn one wrong victim and the Powers of Ravenloft will give you their evil blessing. People don't tell Strahd's Minions to leave the beggar alone. They don't go out to hunt the monster. Tehy lock and bar their doors instead, unless they feel they have no other option than to fight. Because evil may go away and eat someone else, but it will eat me if I raise my head. Tall poppy syndrome is rampamt in ravenloft. Thus, a tearful story how you stoned that caliban in your home village only to wake up the next day malformed with horns and scales just like him will probably keep the pitchforks off your back as long as you BEHAVE like a human, not a monster.
For that reason, I think it is feasonable to have the new races in Ravenloft. The point is, it will be a hard secretitive life for the new races, and they have to keep their abilities hidden, just as wizards, warlocks, paladins etc. will in most domains.
It may be another question if it is worth playing an Eladrin if you can only teleport in exceptional circumstances, but that is for the player to decide. As a player who loves playing e.g. Doppelgangers, I can tell you that is is sufficient for me to know that I can change my shape to really like my character's race, even if circumstance dictate that I in practice always carry the same face. Many players are drawn to a specific race by their flavor, and it can be rewarding to allow them to play it, even if it is restrictive.
Also keep in mind that Ravenloft can mean very different styles of play. A dragonborn may not fit into a real Gothic Horror story, but then again, the very concept of a hero may not, either. In most Gothic Horror stories I read, there weren't any real heroes: The villans were brought down by fate, by hubris, by their own designs. Like a quote in one of vthe Ravenloft Source books says: Gothic Horror is like a primitoive detective story where Fate, or God, play the rolem of the detective.
The "heroes" were just like spectators, watching in horror as the tragedy plays out.
That's not how most play Ravenloft, so be ready to think what Ravenloft means for you. Gothic Horror? Survival Horror? The Hunchback of Notre Dame, Silent Hill or Resident Evil 4? There are many styles of play possible in Ravenloft. The new Races may be impossible in some and not in others.
