The problem with sticking to 12 is that it's not really an option any more. You lose out on newer web technologies, and at least on Windows you run into security issues, since enabling newer TLS versions makes Opera prone to crash.
https://gist.github.com/wchristian/9a479b6969ba653681f50b415...
It is in practice entirely viable to keep using Opera as the main browser and punt troublemakers to other browsers as necessary.
That aside, the SSL/TLS issues alone make it unusable. Changing the settings to make it use the newest TLS version it has support for makes it crash, and even if it did there are no updates at all any more. That means no security fixes, no support for new cipher suites, etc.
This is incorrect in various ways, if only because there exists at least one example of someone using it successfully as the main browser, while also being employed as a web dev: Me.