As a Roblox developer it is currently impossible to save data across a number of unrelated places.
The only other option a developer would have to do something similar is to use a universe - but they effectively ruin the reason developers use separate places anyway.
If a developer wants to create a number of places in a universe, they will have to create a custom system for joining specific games and seeing information about the game. This already lengthy task is also made longer if the developer wants friends to be able to join each other.
Additionally, in terms of groups, using a universe will mean you won’t be using the group games tab like it seems to be designed to do. I would like to see how many people are in a group game and be able to join it from the groups page - but if this game is a universe, you would only have information for that universe.
It is also frustrating for the player having to load a hub place then load another place, especially if they have a slow internet connection.
^ without a universe
^ with a universe
My personal preference for the two examples is the first because you can see how many people are in each place, and can join a specific one without having to go through a hub world. This saves a lot of time and development work.
As part of a vision for a group I am working with, they want to have some sort of currency and skin system that is in all places. This means every group place has to be merged into the universe, and a complex teleport and information system needs to created.
Why would someone have to do all of this to reliably save some data across places?
If this issue is addressed, developers could use data saving to its full potential to create interconnecting places without restricting access to a single place. Users - groups especially - could have meaningful systems that work well and don’t cause other problems.