I'm guessing that it was fermented at too warm of a temperature so its full of hot, solventy fusel alcohols.
It is also probably only given 2 weeks fermentation time (or so) so it hasn't sufficiently aged to allow proper fermentation clean-up to occur so its full of off-flavors as as well as the above-mentioned fusels have not been allowed to mellow.
Lastly, because it wasn't allowed to ferment fully, there are tons of yeast still in suspension which can be rough on the digestive system, especially those that don't see alot of fermented foods in their diet.
Am I on the right track here?