In practice, calculating digital signatures of a file's contents is a more robust mechanism than time stamps for determining what needs building. However:
Developers used to the time stamp model of Make can initially find digital signatures counter-intuitive. The assumption that:
% touch file.c
will cause a rebuild of file is strong...
Abstracting dependency calculation into a single digital signature loses a little information: It is no longer possible to tell (without laborious additional calculation) which input file dependency caused a rebuild of a given target file. A feature that could report, "I'm rebuilding file X because it's out-of-date with respect to file Y," would be good, but an digital-signature implementation of such a feature is non-obvious.