On iMessage, your keys are managed by Apple. You effectively fully trust them (which seems to be the assumption in most of Apple products anyway). I wouldn't call this a "real" E2EE implementation.
In WhatsApp, you're limited to one device logged into your account, and the rest are proxied through it. And message backups, those are annoying.
In Signal, you have all those stupid backups too, and while you're able to log into multiple devices (it seems), your past messages don't load "for your own security", and there's also this stupid time component so you get logged out on your computer if you haven't used the Signal desktop app for some weeks (which I don't).
Whereas on Discord, Telegram, Slack and other IM services without end-to-end encryption, you log in on a new device and that's it. You instantly get access to all your messages since the beginning of time, and stay logged in forever.