Opera doesn't have a timebomb but one modern equivalent is "hideous security defect"
...and they have had those. And fixed them quite promptly.
But Opera isn't going anywhere anytime soon. Unless, as I speculate from time to time, a major competitor based in Seattle crashes a plane on Opera's building, or something, and wipes the company out just like that. I genuinely don't understand why that sort of thing happens so rarely.
Also, you shouldn't really develop for Opera, you should develop for "any or every capable web browser or other suitable client, regardless of operating system". But this, evidently, can be your development tool. And even if there is a new and unfixed hideous vulnerability in Opera, you can still safely use it internally for development and testing.
What I'm not clear on yet is whether the open source part works separately from the Opera browser itself. It reads like you could write your own web browser - call it Rigoletto (or is that taken?) - that Dragonfly would plug onto, but it also reads like no one is expected to do that any time soon.
I'm not directly interested anyway, I just like Opera.