Is the snap at Install Nextcloud Desktop Client on Ubuntu using the Snap Store | Snapcraft either supported by the nextcloud project or created by someone affiliated with it?
As far as I know, the official maintainers just provide the AppImage. Beyond that communities can maintain their own version but it is up to them to keep it updated.
There are also the PPA repositories which seem to have more current version than you snap image:
The risk of 3rd party is always that they start it some time and then it gets abandoned. Best is to check the version and when the last things were updated to see if it is still active.
I’m more worried about a malicious package stealing all my data
The build source code is linked to from Snap store entry: GitHub - glasen/snap-nextcloud-desktop-client
The ultimate problem here is that there’s no root of trust - the snap package is developed by glasen but there’s no link from the nextcloud project to glasen that would allow users to evaluate the trust level
So don’t use it? That’s up to. There are half a dozen other ways to install the Desktop client, including official ones. Or build the Snap yourself from those sources/etc.