And now it working because chromium-ffmpeg slots have been changed in snap store.
snap interfaces chromium-ffmpeg
snap interfaces opera-developer | grep chromium-ffmpeg
First of all this works only in Beta and Developer versions.
And then you have a very strict requirements.
Small comment about the Netflix support on Linux, it's not so straightforward to make it work. This is what you need:
compatible widevine libraries (see /usr/lib/x86_64-linux-gnu/opera-beta/resources/widevine_config.json for search paths, version 48.x). I'm pretty sure some distros have a separate package for it with some custom installation dir, please let me know about that in comment
ffmpeg with h264 support (e.g. chromium-codecs-ffmpeg-extra version 48.x)
fake user agent to Chromium (--user-agent command line switch)
This version 48 is determined by chromium version our product is based on, major version mismatches sometimes work, but no guarantee.
In my opinion at least for now it's not worth the effort. Better just use Google Chrome for Netflix.