Lots of advantages. A key one is if you every do switch to index.php or some other file type, all your links to index.html will become completely useless and you’ll have to start from scratch. If you just link to the root url you can switch file types without switching linking in any way.
Besides that you risk search engines viewing these / and index.html as different pages.
There is no reason to link to index.html, so why do it?