It's hard to estimate what the risk is Erica. Most of the cases I found relate to problems with improper attribution. Unsplash is automatically attributed when you download, so that should cover any issues, as long as it was legally uploaded. It is a bit of a nightmare and hard to know how worried to be.

Someone dropped this story into the comments, which is interesting and suggests that the bulk of the problems are around attribution. https://medium.com/@doctorow/a-bug-in-early-creative-commons-licenses-has-enabled-a-new-breed-of-superpredator-5f6360713299