Some of the adblock issue is possibly an engine problem (there's some pre-script hooks that Webkit-forks don't support).
Password autofill is something I want to take care once there's a plugin API, but there are ways to make it work (the password_fill userscript, autotyping in KeePass*).
Referrer control unfortunately isn't possible with QtWebEngine's API for now...
Zotero won't be possible, but I guess that won't exist any longer with WebExtensions either?
And Greasemonkey support is on its way: https://github.com/qutebrowser/qutebrowser/pull/3040
https://www.zotero.org/downloadbookmarklet
I'm glad that Greasemonkey support is coming, though. Will the GM scripts run in the same JS scope as page scripts?
Webkit forks also render text slightly worse for HiDPI screens and don't seem to support CSS `hyphens: auto`. Which is annoying because I really like justified text.