There is no evidence. safety is about staying multiple steps ahead and risk mitigation when possible.
Centralized servers are a single point of failure that could be compromised in the future.
My contact's devices they use signal on are insecure and could be easily compromised in the future.
SMS/Cell network in general are insecure as hell and I avoid it as much as possible.
Why would I expose all that sensitive data when there's literally no need to? Simplex works great for me.