I think it's more a practical concern than something that can be proven conclusively; the behavior characteristics (e.g., collision probability) are known for a

I think it's more a practical concern than something that can be proven conclusively; the behavior characteristics (e.g., collision probability) are known for a given hash function, and if you're implementing HLL, you choose an appropriate one based on your use case (expected cardinality, etc.) Redis uses a modified 64-bit MurmurHash2, for example.