I introduced git at $DAY_JOB roughly 2 years ago and based it on submodules. We use the submodule subscription feature of Gerrit to lessen the pain [1]. So submodules are not that bad but I really believe they are trying to solve a problem better solved by a dependency management system (maven and similar) instead.

So even subtree might be "easier" you should really try to solve your dependency problem using a resolver instead of your version control system.

[1] https://gerrit-review.googlesource.com/Documentation/user-su...