While it’s a generalisation, I think most of us in the web development community have an internal control freak just lurking inside of us. After all, there’s so much that can (and generally does) go wrong. And you know that it’s always the one thing you let go that’ll come back to bite you.
With Flash, you own the experience. Every pixel and its exact position are determined by the designer. HTML 5 is different. With HTML5, you relinquish at least some of that control to the browser. This can be a bit of a culture shock for some people.
Personally, I don’t think this is anything to stress about. While it means you may not have complete control over the experience, it should also mean you’re able to deliver a more tailored experience to your users. Primarily, this is because the browser understands their current, situation and context better than someone working in a dev shop somewhere.
A good example is video. In most desktop browsers HTML5 video will play and look how you style it – it will be the size you specify and will inherit the CSS properties you apply. But then there’s mobile. On a mobile browser it’s more likely that HTML5 video will play full screen and inside the phone’s native video player. It’s the same video but the experience now suits the context.
Ultimately, this means that users get a better experience and the designer does not have to have amazing powers of foresight to predict every last context their content may encounter. Now that’s got to be worth giving up a little control.