So here's little feedback from my side:

-Steam friends don't show up for some reason
-There is no option to 'unlink' a platform properly. You can disconnect, but my steam games will still show
-No option to manually link the same game that exists on two platforms if the merger is not done automatically
-the automatic scanning of a folder for gog games is unreliable. Manually scanning a folder will add a game but settings are often reset upon closing the client
-adding a gog game manually (that hasn't been recognised) will start the updating process even though automatic updates are set to disabled in the settings
-a heavily modded game like quake for example that isn't recognised by gog can't be added to the client either without the game redownloading all files and overwriting
-bookmarks should show numbers of contained games
-there is no feature to backup galaxy settings. if I reformat my system, everything is gone, all the painful integrations etc.
-constantly changing background colors is annoying in my opinion
-selecting a bookmark the last filter settings don't show up, everything is reset again

Also in the friends list on the right side ? This is a know bug, usually resolved by connecting and then disconnecting again Well, you could either make a backup of the Galaxys "ProgramData" folder or rely on GOGs Serversync:

What gets synced with gogs servers

- Custom cover arts
- Manually added games (you will need to re-link the games once in a new PC or installation, though, but they're still on your list)
- Games you own on every platform
- Tags
- Edited metadata
- Hidden games
- gametime for manually added games

What does not get synced with gogs servers

- Bookmarks
- Integrations