What is the exact reason that one should not cache the vendor directly

[Ilyes512]

The readme mentions the following:

Please do not cache vendor directory using action/cache as that will have side effects.

I would like to know more about what is exactly the issue? What are these side-effects? I currently do cache the vendor dir, since I didn't came across the line in the readme at the time when I implemented it. So now I am wondering what the reason is as it does look like it works just fine.

(I will try and benchmark the difference between caching and not caching the vendor directly.)

[shivammathur]

I think that is based on this discussion.
travis-ci/travis-ci#4579 (comment)

But, If you publish composer.lock file and the cache key is based on the hash of that, I think it might be fine to cache the vendor directory now. If anything goes wrong, you can evict the cache easily using the actions UI.