This is particularly true for businesses that offer things like yoga classes, for whom the marginal cost is ~0. This is obvious to everyone now, but early on most small businesses didn't think of Groupon as what it is: a form of marketing that requires a fairly particular cost structure to be cost-effective.

One problem early on was Groupon wouldn't allow the limitation of numbers of "deals" sold. That put some people out of business. The people running Groupon were acting well beyond the bounds of morality. IIRC they did correct that after some bad press.

The money lag also killed some small businesses who had assumed they'd get money at the pos to cover their costs and hadn't accounted for increased custom needing greater spending on inputs when money was locked up for 3 months. Cashflow interrupts easily kill a small (micro) business.