Heroic is an impressive achievement. It just isn't a full replacement – and most of these points I'm about to list aren't things it's trying to or should necessarily do as a games launcher. Off the top of my head:

• It doesn't work for social aspects like friends, statuses, etc.
• It doesn't work for monitoring achievements.
• The storefront is just the actual gog.com webpage rendered as a surface. GOG Galaxy's store UI, by contrast, flows with everything else.
• I don't think games auto-update, although I could be wrong.
• It's bloated by nature of also being a launcher for Epic and Amazon – platforms I will never use. GOG Galaxy allows crossplatform stuff, but it's not a full-on multilauncher.
• The UI is pretty ass. I sympathize a lot with this one as someone who works on the (often disastrously undercoordinated) UI of a similar-profile project.
  • Rectangular UI elements' corner rounding is all over the place (from sharp 90° to Material 3 and everything inbetween).
  • Themes are extremely samey with an enormous bias toward dark themes (I say this as someone who exclusively uses dark themes: a single light theme and thirteen dark themes means you don't give a shit).
  • You can't hide the left-hand menu bar to actually center the page you're viewing.
  • Actions like toolbar dropdowns have no animations (I understand not wanting these; that's accommodated with a "Disable Animations" option).
  • There's absolutely zero compatibility with Orca (screen reader) that I can find.
  • Etc.

Again, all of these except the UI aren't things Heroic is doing wrong or even supposed to be doing at all.

Side note: in Heroic, the GOG storefront opens with UTM parameters in the URL for "adtraction". Wonder what that's about.