When I spoke with the Viator customer representative on the phone they did not tell me that this card guarantees you nothing. It only gives you a chance at doing the various activities if they have room or inventory for you. Everything must be booked in advance, and individually.
I think you need to purchase the Go Cards wherever you travel, they are a real bargain and encourage you to see as much as you can.
We purchased two of the 7 day passes for 285 each. I did my research and made a schedule of the things we were going to do each day. We did nearly everything that interested us on the card and saved 368 per person. You need to make a schedule prior to purchasing the card to make sure what you want to do will save you money. If you just buy the card and aren't organized you will end up missing many activities and not getting your money's worth out of the card.