IT appears to work only first run in a new bottle.
More than half of what is offered through Prime Gaming is add on content for other games, and those can be claimed through the website. The rest are mainly browser games, vintage SNK/NeoGeo ports, and a few "limited time" offers. Overall, probably not worth the effort.
I was lucky and grabbed a title that just happened to be distributed in a ScummVM wrapper. Won't work under CX, but the data files work perfectly under the native ScummVM app :)