Incremental builds with the said tools (Please, Bazel) should be a non-issue due to emphasis on hermetic build from these tools.
My perspective here is specifically from that of a maintainer of large builds where the code base is always under active development. Murphy's law becomes your enemy at scale.
This is not a criticism of Please - just an observation that as an industry we are not there yet. But, I'm happy to see things moving in the right direction.
genrule(name = "oops", cmd = "date > $@", outs = ["oops.txt"])
Again, I am not picking on Bazel here. Correct builds are difficult enough to get right even when everyone involved wants to keep the build correct! But, the reality is that a lot of real-world build conditions can be downright hostile. (did you know that ext4 vs xfs can change the order JARs in the same directory are loaded off the classpath? I wish I didn't!)