Well let me go down the list
1. It uses pre-hopped malt extract for fermentation, which means no variability in styles of hops, IBU's, flavors, aromas, anything you could easily achieve from normal homebrewing.
2. You have to follow their specific recipes, being that all the ingredients comes pre-packaged in a single can. You can not alter any of the recipes being that there is no way to add any sort of specialty malts, sugars, spices, anything, because you are never mashing and you are never boiling.
3. From the video I watched on the home page, it looks like they have 3 recipes, and only 3 recipes.
4. It uses dried yeast that hasn't been rehydrated. Now you could go out and buy others, but almost no homebrew stores sell any dried yeast other than generic ale, lager, and wheat beer yeast.
5. You are restricted to making 23 liters at a time, which equates to around 5 gallons. BUT, lets say you drink all your beer, now you have to wait another 7 days until you have more beer to drink, because its the same fermentation and serving vessel.
4. The INSANE INSANE cost. Starting price on the homepage of the website is $5660.00. You can go to the store, buy yourself a proper homebrew kit with ingredients for around 100 dollars. In my entire setup I have, where I can literally be brewing 8 different beers at a single time, and have 8 others in kegs, it cost me probably 1800 dollars, and thats an extreme setup.
I am sure I can list some others if I look at it close enough, but you get my point.