Counterpoint: As a user, often the only way to get a video to play in any browser on linux is to modify the user agent. And then, when you do spoof the user agent, it works just fine.
Some of our video test grid machines are Ubuntu, so from our end Twitch video should work pretty well on Linux :-)
The browser also included a built-in user agent compatibility list and the user could manually add sites to use a specific UA string.
Most sites would work fine in Opera with the correct UA. Many did not work when using the default UA with the word "Opera" in it.
A recent success story: Mozilla recently added a UA tweak to work around Slack's Chrome-only check for video calls. Slack engineers reached out to Mozilla and, just a few days later, Slack removed their Chrome check and now support video calls in Firefox without a UA tweak. :)
I wouldn't call that a success.
Welp, better spend a few minutes downloading the same thing but in a Chrome sandbox.