There's a myriad reasons why flicker and buzz occur, because there are a number of ways to achieve dimming. But all of this comes down to cost.
If dimming is important to you, there are a number of architectural grade bulbs that offer deep dimming to 5% or 1%.
Personally, for this application, I've had good luck with Cree bulbs of this type at home (on Leviton dimmers). And in the house of worship environment with Cree bulbs on Doug Fleenor dimmers (http://www.dfd.com/dmx8dim.html)
Second that Doug Fleenor recommendation. They make some fantastic lighting kit.
I'd also like to add that looking for lamps with the CEC Title 20 & Title 24 certifications should be helpful in finding bulbs that dim well, as California has much stricter regulations on dimming flicker and buzz than the rest of the country.
I've tried through 5 different MR type LED bulbs and every one of them had an extreme flickering issue. All of them were labeled as dimmable.
I've had better luck with the standard A type bulbs. But only because there were more thorough reviews available.