Opera added a user agent header "selector" pretty early so it would tell the webpage it was chrome/IE/Firefox. It was important for compatibility for a lot of websites. I'd trust that listing less for them much less than I would for the bigger/default browsers.

The migration from their own codebase to chromium in 2012/2013 was...rough. They were the first browser to have cross-device synch and you couldn't import bookmarks for a long time, much less RSS feeds/everything else people used Opera for. Their original userbase took a sizeable hit.