How does mixing different kinds of beers/liquors mask the taste of alcohol?
According to the article Why People Hate Drinking, there is considerable variation in how people perceive the flavor of ethanol:


Some find it is primarily sweet (non-tasters)
Some find it bitter and sweet (average)
Some find it overwhelmingly bitter (super tasters, as defined by sensitivity to tasting a specific compound, PROP or 6-n-propylthiouracil, as bitter)


Depending how people perceive alcohol, it may be easy to mask it, especially with strongly sweet flavors, or when there are other more apparent bitter flavors.  

However, this is as much to do with the specific physiology of the taster as it is to do with the contents of the glass.  The same drink tasted by different people may give differing experiences.

Beware the Long Island iced tea.