This is what vim syntax highlighting from the plugin produces:

(notice "else" branch in "foo" function)

Now let's move plugin's syntax/unison.vim to syntax/unison.vim.bak and look at stock vim highlighting:

Again, look at the same "else" branch.

This is just a small example; for bigger files with plugin highlighting they are mostly in one color. Example:

Stock highlighting:

Unison 0.5.48, vim-plug, vim 9.1.1744, FreeBSD 14.3-p3, solarized8 colorscheme (using "default" does not help)