Mostly the spooky action at a distance. Gitignore expressions and inheritance of the ignore expressions can exclude files you actually need to include, and you basically lose sight of those -- admittedly those are probably safer bugs than including things that shouldn't be included. But if a file (or even specific lines) shouldn't be included, I would prefer the file/lines have a marker that says it shouldn't be included.

I run the same presubmits on pre-receive and merge requests, so you wouldn't get too far shooting your own foot. But yeah, if you're not using the feature then it wouldn't save you so it's not foolproof. I also run tools for scanning for credentials, etc.