Subtree Split has a bug in which irrelevant commits are included in the sub-project's history. Along with breaking compatibility with branches created before the bug was introduced, this significantly and unnecessarily bloats the sub-project's git repository.
I contacted this group about a this bug back in April and later followed up with a reproducible test case. However, I've failed to get a response or traction, despite the significance of the bug.
My previous emails, including the reproduction steps, can be found in the archives: http://lists-archives.com/git/875747-subtree-split-includes-commits-outside-prefix-directory.html
Regards, - Harpreet "Eli" Sangha