It not being the latest version if React is hardly a major issue. You miss out on some of the latest optimizations, but the API hasn't changed.
Which is actually a really good argument for avoiding this project - if it's so easy to stay compatible, then... stay compatible. Feels like a dead project if it's so far out of date.
Speaks to longevity and maintenance however. It’s not like React 15 is exactly new in the space, or hard to update to.