“There aren't many flaws to pick at Nielsen for. I am a member of their other panels, so perhaps I am a little bias. The only difficult stage is waiting to see if you've been accepted. Once successful, they will contact you and continue to occasionally interact with you through emails, calling your phone to check on you when you forget to send them data, annual birthday greetings, etc... It's quite relaxed and casual.
You accumulate points by scanning barcodes from the beauty products you...buy with their app and enter quantities, prices, retailers, and sometimes take photos of new/unknown products. Things to scan include cosmetics, skin and haircare, supplements/vitamins, toiletries, etc...
After you have enough points, they will send you a $10 Flexi egiftcard, where you can then choose which retailer you want. Points will restart after redemption.
Of course they're not pressuring you to make these purchases on a weekly basis. For a week where you bought nothing, you just need to let them know by selecting "no purchase" on the app or "vacation" if you're away on holiday.