I agree that support for Flash in Linux leaves a lot to be desired. At least there is *some* support for Flash, which is more than you can say about some other technologies.
I haven't had problems with Opera and YouTube. In fact, I just now went to YouTube with Opera 9.27 and Flash 7 and played opera videos with Placido Domingo, Franco Corelli, and Alfredo Kraus. No problems. Even video/voice were pretty much in sync. Since I've played many YouTube videos with Opera in Linux and have not had crashes, I wasn't expecting problems this time and didn't have any.
I've rarely had Opera crash at a Web site--probably less often than SeaMonkey or Firefox. All of the browsers have occasionally just disappeared, which is not something I've had happen in Windows. But for the most part they've been quite reliable. Opera 9.2x has never worked well with the MPlayer plugin and for a lot of media, SeaMonkey or Firefox is a better choice.
My slowest computer is a 1.3 GHz Celeron with a gig of RAM, so I'm not dealing with low RAM or a truly antique processor. That probably has something to do with my relative lack of problems.