Section 7.3 of SP 800-108:
To comply with this Recommendation, the derived keying material shall not be used as a key stream for a stream cipher.
The level of security provided by using the key derivation functions specified in this Recommendation to generate a key stream for stream ciphers has not been investigated.
I was wondering why the derived keying material can be cut into valid cryptographic keys but cannot be used for stream cipher. Since it is marked as a shall not, are there any serious concerns if the derived keying material is used for stream cipher (other than it may not be long enough for given plaintext)?