[GTP]

Negotiation makes the protocol more complicated, so more room for bugs. Additionally, sometimes (or maybe all times?) you need to explicitly configure a client or server to not accept broken algorithms during the negotiation, so you would need to keep track for yourself for which algorithms are safe to use and which are not. Anyway it would be interesting to look at how the Wireguard folks would handle this, as Wireguard is a protocol with opinionated crypto.