What have you been smoking? HTML5 is certainly a security risk. Open up new functions in the browser and you open up new potential vulnerabilities and risks.
That depends on the browser. If you're looking at a browser like Opera, Chrome, or FireFox that limit the abilities of a web page to access the ability to run shit on the PC then Flash -- which by being a seperate program can circumvent such limitations -- is considerably greater of a security risk than HTML5 is.
Depending on the browser and security settings, of course. But once a plugin fires up, your security settings mean jack.
HTML5 is not better quality video. Where did you get that idea from? Steve Jobs' ass? (kidding) If anything Flash gives extra features over HTML5 to improve quality.
I get that from my own viewing. The video since I switched to HTML5 has had better audio and video over when I was using Flash. Notably so.
Scribd is running extremely slowly now on around 61% of the internet browsers (IE). In Opera layout is poor. I think only Chrome displays that site properly, didnt test it on FF. Not to mention it's not HTML5
Except it is HTML5, that page doesn't even say it's not HTML5, they just say its crappy HTML5. Fairly new markup language, you should expect a lot of it to be a coder's nightmare for a while. Hell, there are HTML4 sites that are as they describe. . . And it displayed fine in FF when I was looking over a bud's shoulder. Opera does have a conniption with it. It looks fine on IE8, but as you said runs like poo.
New things do this, this does not make them sub-par because people are still figuring out how to use it properly and improve upon it.