1. Yeah, hash functions are deterministic, but your input needs to be determinsitic across machines too. For example, on a unix system, you may want to conditionally build if any files have changed. To do that, you could generate a deterministic hash of the dependencies with something like `find . | sort | xargs sha1sum | sha1sum | cut -d ' ' -f 1`. Including mtime in that would break across machines.

2. Mainly because doesn't translate across machines; it only applies to your machine. If someone checks out the repo on their machine, mtime is different. As soon as you move a build system to CI, you need something better than mtime, like content adressable hashes, if you intend to cash targets.

3. It executes the Walkfile twice for each target in the graph; once to determine the targets deps, and once to execute the target. This definitely hasn't been a bottle neck for anything I've used walk(1) with so far.

4. Correct! But even more so, replace "shell script" with "executable". The Walkfile can be written in any language you want, as long as it has the executable bit set.