From wp-cli/extension-command#62 (review):

Have a PR to expose PHP notices/warnings when running behat which I'll post one day rather than having to do ad-hoc debug file requires and tags etc...

One caveat when doing so: we may expose a good number of issues we're not currently aware of. Before we commit to this, it'd be good to run a complete test suite with this enabled to see what sort of existing problems we have.