Maybe I should put in a whole paragraph of qualifiers and disclaimers, but the response wasn't really aimed at people not having a problem rather at the large number of people who are.
I am referring to "normal" beer 1.040-1.050 and there are a lot of brewers who want to dry hop injudiciously and who are having problems.
I have two major concerns with extending fermentation times
Infection I doubt that anyone's brew is a monoculture of just the yeast we want, what protects a beer from unwanted bugs is the conditions at the end of the fermentation process (high alcohol, low oxygen, low residual sugars, lower pH, high pressure once conditioned, hop products...) the quicker you get a brew from the start to the finish the less chance any of the nasties have of taking hold.
Yeast breakdown products eventually yeast will break down and it will affect the flavour of the beer, I know there has been a lot of discussion about this subject recently. Extreme yeast autolysis takes a very long time or very adverse conditions but there are a lot of undesirable flavours I believe can be ascribed to yeast management short of "Burnt Rubber" that have a negative impact on the beer.
In my experience and based on a lot of reading big healthy pitches and dry hopping early are two important steps to preventing unwanted flavours.
Mark