You are giving people a _ton_ of trust when cloning a repo or pull request that has a .envrc which will be automatically executed by direnv. This file could do anything from steal your SSH keys to delete your entire home directory or worse. And the only thing preventing that from happening is the suggestion of "oh be sure to eyeball the .envrc file and make sure it looks ok, then run this command to enable it". People will just google for the first "how to I unblock .envrc file" and blindly run the command, then oops. Or a determined attacker will just obfuscate what they're doing so the .envrc looks fine but you failed to realize some esoteric bash-isms were actually invoking an obfuscated script and... oops. It's just begging for a supply chain disaster on a similar scale as npm's infamous issues.
VS code and some vim extensions have similar issues with automatic project-specific configurations that allow arbitrary code execution too, and neither have a good solution beyond 'just read all the code and understand it to make sure it's not going to hurt you'. It's not really a unique thing to direnv.
We really need a better model for collaboration and consistent dev environments. I think containers and docker/chroot/etc. environments with explicit opt in of your local resources, configs, files, etc. are a much more sane and security conscious path towards it. A script executing to setup my python environment for a project should never, ever have access or even know about my personal password store database for example (unless I explicitly decide it should).