We've used submodules pretty extensively in the past but we're working to get rid of as many of them as possible. A friend and former colleague liked to say they were the worst solution... with the exception of all the other solutions. Practically speaking they are probably more confusing to new developers who haven't had a lot of experience with them. We would often run into problems that stemmed from someone making a change in the submoduled repo and then forgetting to update the ref in the parent. Or people would navigate to the submodule to do the work there rather than checking it out separately, and then be confused over the detached HEAD status and check out a branch (not a problem, necessarily, but confusing to some extent). For some of our dependencies we've gotten rid of submodules in favor of hosting internal pypi packages.
