When I see warning like this, I feel strong urge to try it just for sake of what could possibly happen.
Python Cryptography library in Primitives section if anybody wondering.

Read about /dev/random and entropy on *nix systems... It's probably using /dev/random and depleting the entropy pool, which can lead to serious problems.

@D3add3d Good to know. I'll investigate further why they are claiming that, but this is definitely good guess.

@D3add3d I don't think they do (at least as far as I know).

Many developers not experienced in cryptography tend to use primitives incorrectly or in an insecure manner, creating horrendous insecure systems (e.g. by using ECB mode, not using MACs, static keys, unauthenticated handshakes, insecure algorithms and much much more). I think this is the reason for the warning.

@sbiewald that could be another reason but my first guess was that since it is something that is dealing with cryptography it will also generate some keys and for that it most likely needs randomness