Flash is the only piece of software I use that crashes constantly. As in nearly every day. Compared to once, twice or even as much as – gasp! – a dozen times per year when using all that other stuff I use.
It’s disruptive. It’s annoying. More so than spinning fans. That’s a minor annoyance compared to the crashes.
Flash may (currently) have more capabilities than browsers, but when matched point for point (e.g. video vs video, SVG vs vectors, Canvas vs procedural bitmap generation) I've not seen any evidence of spectacular performance differences, despite browser code running in a secure sandbox. And when there is it's rarely something fundamental, like you'd see comparing HTML5 or Flash video against a simple video playback system, just bugs and/or unoptimized code.