I still miss the old Opera, including the built-in email client I used. So yeah, I agree.
Honestly, Opera is one of the best pieces of software I've ever used. It had tons of features and customizability without being slow or feeling "bloated". I never quite understood why it failed to gain marketshare 10 years ago while Firefox did.
If anything, using the MUA bundled with your browser was less power-userish than using a stand-alone client.