WhatsApp works properly on GrapheneOS including end-to-end encryption. I don't recommend using it but you are better off running it on GrapheneOS than AOSP or the stock OS. Signal has comparable functionality / usability with better privacy / security and there are other options.
Conversation
Replying to
do WhatsApp and Signal work less reliable on Graphene OS due to lack of FCM? do they consume much more power because of that?
1
They work without it and they're not unreliable. Having multiple apps managing their own push notification connections uses more power than having a shared connection but it largely depends on how many apps are being used. WhatsApp has an efficient implementation, Signal less so.

