It is entirely possible investors will get burned in an extraordinary fashion on Groupon. It is also possible they will end up with money hats. Capitalism happens. If capitalism happening to you would negatively impact your ability to feed your family, do not buy stock in Groupon. (Do not buy any individual stocks, either.)
With regards to their accounting practices: I'm agnostic as to whether any particular treatment of another company's numbers is maximally reflective of the interests of third-parties wishing to invest in them. That said, have you noticed we are not exactly a paint-within-the-lines industry?
Heck, even answering very freaking simple questions with intent to be maximally honest while complying with all regulations gets very difficult. For example, when do you think you can recognize revenue for selling someone $10 for 1,000 gold coins if they then immediately spend 500 gold coins on a Sword of Dragonslaying? If you have not read about this specific case before, I guarantee your first three guesses are wrong. Or, to pick an example near and dear to my heart, what exactly is getting sold when you take $29.95 from a customer and mark their account as Registered? Is it real property? If so, where is that property sold? "The location that a customer takes delivery", swell: do they "take delivery" at my business or at my server or at their home address or at their place of work? Is it perhaps not a real property and instead royalties? Is it a fee for a service? Oh, that depends on whether customization and support is offered: how much support is support? Answering emails is support if I make changes to the product on the basis of them?
I have given tax offices on two continents head-explosions just with the accounting for BCC. This stuff is hard.