The Groupon business model reminds me of timeshares (covered well by John Oliver recently). You supposedly get a good deal on something you can use in the future, and then it is made as difficult as possible, in my experience, to actually use it.
Then it was great, except only for regular-priced options and not available on weekends or holidays.
Then people realized that, instead of getting a Groupon for a new cafe or restaurant or kickboxing studio and then becoming a repeat customer, they could just keep getting Groupons for a new cafe or restaurant or kickboxing studio every week and never have to pay full price for it again.
Then actual regular customers got fed up with the businesses getting a huge influx of entitled assholes who didn't care about the place or the people, who all rushed in on the last possible day and demanded that they all get their booking that they waited 8 months to redeem.
I loved it at first and tried out a lot of new places that I continued to visit, and some that I didn't, but after the honeymoon period it was just an absolute disaster for everyone - the businesses, the existing regulars, and the people who realized that they paid money for something they won't have time to use.
I've heard it said many times that the worst customers are the ones who've paid the least.
I suspect there are a number of Internet businesses--this, eBay (in its initial flea market incarnation), Kickstarter, etc.--that in their heyday were something sort of novel and trendy. But, over time, a lot of people probably discovered they were buying things they didn't really need and it was no longer fun as sort of a game.
Some like you said, are basically an reservation to for accomodations for a specific time period every year at a specific place.
But there are others that are basically "You have 4 nights a year at accommodation options X, Y, X". So you aren't guaranteed any specific locations or days ahead of time.
What GP said is true if it's a single place. Those are typically bad because similar accommodations tend to be cheaper, the timeshare tends to depreciate, and the market isn't very liquid.
The ones that offer you nights per year are hard to redeem because they intend to sell to capacity. If it's a single unit, they want to sell all 52 weeks out of the year. You're typically competing with all the rest of the buyers to get a "good" week like Christmas or Thanksgiving. That can also tend towards being a bad deal if you're paying something like the price averaged over a year but you can only get in during the off-season when it's too cold/hot/rainy/etc.