Strange you got no reply.
You are correct, as I am facing the very same issue.
Opera 27 brought a number of annoying issues, for example some password saving didn't work, current Adobe Flash plugin flagged as outdated, and this silent install bug.
And since Opera went Chromium, there isn't any decent documentation of silent install switches.
For example, set default browser does not work on Windows 8.1, as it only triggers the OpenWith dialog.
You still have to click to set the default browser.
Cheers.