Barleyman: Fully customizable seems to be a secret phrase for "Rubbish as installed unless you spend quite some time figuring out themes so it's not ugly, how to get playnite pick up artwork for epic games and so on". They don't support multi-threaded updates so if you have humble there, it'll clog up the whole thing for quite a long time every time you start it. Also as I mentioned the artwork issue (why doesn't this work in standard installation? Is that somehow acceptable?), you have to then rebuild your library with correct artwork.
Playnite also has (potentially) better multi-pane usage .. But the default pane widths are far too wide and waste a lot of space. Again, this should be reasonable out of the box, not expect everyone to figure out the correct place in the settings to apply pixel widths to columns.
Gog Galaxy on the other hand, you install and you go as soon as you're done with connecting the accounts. Exactly what I look for in a multi-platform launcher.
The one thing I'm not impressed with is how long it's taking to fix Ubisoft connect. There's a fix posted in the forum and the latest update wrote the patched python file with a broken one..
As much as i consider your criticisms valid, the person who created the topic had issues with library management. One question was if Galaxy 2.0 was a good starting point. Most of the replies pointed out the issues.
I suggest PlayNite as, IMO, is a very good software for exactly what the author wanted it. Not only the person tested, but also was very impressed by it. I'm sure he / she will fiddle with it trying to make it more usable.
GOG Galaxy still has tons of issues, integrations getting disconnected, reports of slow browsing on big libraries, lack of updates and basic features. Anyway, i'm just glad a suggestion i made helped someone.