Well, there are certainly styles that would be harder to brew as all-extract or extract with specialty grains. For example, as far as I know, there is no one selling a Vienna malt extract, so you'd have to do a partial mash to make a Vienna lager. But, generally speaking, if you can find an extract for whatever type of malt you're looking for, you can make a beer that's every bit as good as it's AG counterpart as long as you have good brewing/fermentation/sanitation practices.