>Although it’s simple, there are serious problems with this method. First and foremost is that dependencies are only rebuilt when the user explicitly requests it; if the user doesn’t run make depend regularly they could become badly out-of-date and make will not properly rebuild targets. Thus, we cannot say this is seamless and accurate.

Not so fundamental, if you quickly make depend after changing includes. Why is that messy? I never got that "buy magic automation, sell simplicity" trading. Deps changed, deps have to be updated. Once updated, they just work.