In terms of helluvalot less critical - is it really though? Remember that the app on your phone is also witten by them, closed source and does whatever they want with your clear-text messages. If the trustworthyness of a messaging vendor is part of the critical-ness question, e2e encryption does not add anything: Either you trust them and could also do so when they process your message on their server, or your don’t and they could indeed spy on you on the proprietary client app.

End 2 end encryption is only a real benefit when the ends actually belong to the user, i.e. theres transparency about the ends being clean, which can only be shown for open source ends. If the ends are potentially compromised, there’s so security / privacy guarantee.