It is the same people who say Git is unseasonably hard to learn. It isn't. Git and its command line is ugly and hard to remember. Understanding what those commands do at a commit and a branch level isn't really difficult. Even the internal object model is only moderately difficult.

I say this as a person who strongly dislikes many many aspects of Unix and Linux due to bad design and terrible UX. Git has a better design than any Unix program you get.

Submodules work okay. It is just Git LFS but for Git repos. Get over it.