It's not to say that using deterministic random numbers is wrong. Rather, when all parameters in a signature system are deterministic, the system is secure. Even compared to a random signature system, the former has some safer features, like always generating the same signature for the same message. However, when part of deterministic random numbers have been compromised (typically caused by engineering implementation and use ill-considered), that's where the nightmare prevails. The residual deterministic random numbers can no longer guarantee security, and even can be exploited by the attacker who via duplicate signatures eliminates deterministic invariants to attack the whole system. The attacking towards Ed25519 and Schnorr signature algorithms can partially prove this thought.