Pretty good so far, but could do with a few improvements...

Galaxy's RAM usage is a bit higher compared to Steam client from what i've seen on Windows Task Manager, could stand to be a bit lower.

Galaxy doesn't seem to have a copy/paste function.

Galaxy only seems to allow the broken Windows version of Earth 2140 to be installed and not the working DOS version.

And the big one, Galaxy's "Play" icons need to be able to have the target directory and program edited (much like regular Windows icons).
Example: I was able to import my exisiting Quake installation including the Quakespasm mod and others just fine to Galaxy, but Quake's title screen demos no longer work. This wouldn't be a problem if I could point Galaxy to the launch shortcut the game was using by default, but Galaxy is stubbornly bypassing the launch shortcut and going straight to the exe instead which doesn't allow editing for command lines that would solve the problem. This is kind of a problem for those using mods in their games.

Other than those immediately noticeable things and that last one that needs addressed, Galaxy seems fine so far, though I have yet to test multiplayer.

As for Earth 2140, the DOS version is bundled as an extra (similar to the manual and soundtrack), while Galaxy only installs the main game and expansions/DLC's - and though I don't have insider knowledge, I would bet that Galaxy can only handle a single "main" game per product and platform. You can download the extras/goodies, but Galaxy doesn't treat them as installable.

In order to have the DOS version be installable through Galaxy, it would have to be its own product (similar to how the original Baldur's Gate is bundled with the Beamdog edition, or how Arena and Daggerfall are bundled with all other Bethesda games, it would get its own entry on your shelf).

I don't know why Earth 2140 is handled that way, but there is probably a reason for it.

Oversight I would assume. Alone in the Dark 2 & 3 were officially only offered as "bonuses" when the first game was released here, and they've now been added as separate installers under Galaxy. I wonder if GOG could fix this or whether it would require some legal eagle wizardry with TopWare...?

Wishlist to add a Launch Options and Command Line Arguments feature in Galaxy...

https://www.gog.com/wishlist/galaxy/set_launch_options_command_line_arguments_for_games

Aren't quite many other games handled the same way, with the alternate or classic version of the game being in the extras? At least e.g. Broken Sword 1-2, Stronghold HD and Stronghold Crusader HD.

I guess with all those you can't install the alternate (unsupported) version with Galaxy?

Correct, on both counts. And that is likely the reason, they are offered for those who prefer them, but they are not supported.

As for Alone in The Dark, which Jamyskis mentioned, those are officially supported, the sold product is even called Alone in the Dark: The Trilogy 1+2+3, thus the product became a bundle with a "sub-product" for each of the three games.