True. But

1. The luajit documentation basically just had a list of features. AFAIK there isn't any documentation that combines the 5.1 reference with luajit extensions (including things that were backported)

2. In some cases, for example Neovim, luajit extensions aren't guaranteed to be available. It just says there will be a lua runtime compatible with 5.1. Which means a truly portable neovim plugin can't use those extensions

3. There are features from later lua versions I would like to have (in particular <const> and <close>) that will probably never get backported.

4. Some features require luajit to be built with special flags