Yes, website compatibility is one of the major issues of Opera. The second thing is lack of support in terms of add-ons and such, but I don't see how they can help that. This might just get more people to use Opera as their desktop browser though. Every time I test a website on Opera, I'm like "I should really give this browser a chance". But every time that I try it for an evening, I notice the shortcomings in support (by websites, add-ons, etc.).

Back then I was used to having fifteen add-ons in Firefox, but after Mozilla made one bad choice after another, I moved to Chrome and live by Adblock Plus and some own userscripts and bookmarklets now. Perhaps Opera isn't as much of a culture shock anymore to use, especially with the Webkit engine.

I do wonder if they considered Mozilla's Gecko engine. It has a smaller userbase I think, but does that automatically mean it wouldn't be the right choice? They don't say a word about this.

All in all, I do applaud the change. It's a hard step to take after you've been working on your own engine for years, but they've done it, and it will probably add a lot in terms of website support.