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.
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do WhatsApp and Signal work less reliable on Graphene OS due to lack of FCM? do they consume much more power because of that?
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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.