You can figure out ways to do it, but it's pretty kludgy (messages addressed to you have to be encrypted with the keys of each device you've authorized) and you can't scroll back prior to when a device was authorized and its key was communicated to other participants, which users might not find convenient.
But it's why I don't use Whatsapp as well, personally. I prefer the cross-device seamlessness of Hangouts (and FB Messenger) to E2E crypto.
Apple seems to do OK with iMessage and it faces the same constraints.
Always had an impression that iMessage is not end to end encrypted.
What isn't encrypted is iCloud backups, which are what provide the ability to scroll backup on new devices.