This might not be exactly what you asked for but I usually don’t make it too technical. The moment you start talking about features and privacy, people will subconsciously start to defend their current behavior and try to argue with you why WhatsApp is good enough.
In reality people are usually somewhat aware of the privacy implications of using Facebook products.
So I try to make a bit of a stricter but more subjective argument that one can’t really argue with: If you want to talk to me, do it through Signal; I only use that.
It’s completely subjective and it doesn’t create a “But, uhm…” situation. I know it doesn’t work for everybody, and you can try to phrase it more diplomatically than just that, but it’s the most honest statement I can give people