There are none.
Halle Bailey is an incredibly talented young actress and I'm excited to see what she'll bring to the sea. Disney is a multi-million dollar company and they wouldn't hire someone for as large of a role as this who wasn't more than capable for the job. In fact, the move that they're including more people of color in the faces of their company increases the quality of the company as a whole. Disney's characters are meant to give young children someone to relate to and someone to look up to. Princesses more than any other Disney character are meant for this purpose. White children have grown up with generations of characters that look like them. It's time to allow for black children to get the same representation that white children have consistently gotten over the years.
It is to no disservice of the story of 'The Little Mermaid' to have a black actress playing Ariel and in fact, it broadens the audience of children that feel close to this story. Movies like 'Aladdin,' 'The Princess and The Frog,' 'Pocahontas,' and 'Mulan' are very important to have someone of a specific race playing the princesses in the movies. It's important to the story. In 'The Little Mermaid,' there is absolutely nothing that says that Ariel should be any race other than the fact that she was originally portrayed as white. Even that isn't relevant to the story and casting a black actress does not diminish the story in any way.
Screw your ideas of what Ariel should look like based on what she was made to look like in a prior ANIMATED film. By bringing this movie to real life, it's about time that Disney's characters reflect real people. Choosing Halle Bailey for the role of Ariel allows for Disney's portrayal of real-life to reflect actual real life. I can't wait for black children to be able to grow up with Ariel as another person to represent them on screen.
If you have a problem with this casting decision, then you are racist. Plain and simple. You're not upholding tradition unless that tradition is that of the values of the racist people who have perpetuated racism throughout the history of our country. Decide now: will you be racist or will you be excited for all of the kids who get to see someone like them represented on the big screen. I, for one, can't wait.