Property settings in Bulk mode - difference between List section and Contacts view
In the "Lists" section, the option "append to current values" does not exist so that instead of adding an extra value, (e. g. the buying role "decision maker" to a user that is already classified as "power user") pushing the update will replace the "power user" value instead of appending the "decision maker" role.
If you open the list in the "Contacts" view, you have the choice to select between replacing and appending which much safer.
Please provide the "append" option also in Lists to be consistent and help prevent users from accidentially replacing instead of appending. The append option is also available in workflows when you set property values.
As an extra security feature, I would love to see that you also add a warning message if a user selects to just "update" and make sure the user is fully aware that all other values will get replaced by the new one.