Darvond: Another thing I noticed Heroic can't do: Dosbox.
That might be true, not sure. But if Dosbox is not native, then can't you run the windows exe inside a Heroic wineprefix anyways? I feel like I'm missing something here :)
dtgreene: Can it handle Wizardry 8? (Lutris fails to run this particular game, at least for me.)
I don't know, but unlike Heroic, Lutris includes custom fixes and tweaks (mostly which DLLs to override and which components to install, based on community experimentation) so if it didn't work out-of-the-box with Lutris, it most likely will not work in Heroic given the same wine and dxvk version. Yet with either one, it is easy to configure everything to your liking, and with some troubleshooting you might get lucky. It's certainly easier than starting from scratch with a fresh wine prefix.
dtgreene: Also, if you've already downloaded the installer from GOG, can you use it to install the game and have it be treated as a GOG game?
I'm not sure I understand your use-case here, but you could run the installer in your previously made Heroic wineprefix, and install the game wherever, then import that game into the Heroic game library and point it to your desired wine prefix (could be the same one, doesn't have to be).
How does this differ from using your system-wide wine to install from the offline installers, and then importing the game in Heroic? Probably not at all. Though, come to think of it, some redistributables might get nicely installed as a dependency with the first method, otherwise they would go into your default wine prefix and be wasted. Hm. Good point.
I find the good things about Heroic are
1. Nice looking game library
2. Extensive wine prefix configuration settings
3. Fast and easy installation of GOG and Epic games
Heroic bypasses the offline installers, which makes it fast and convenient for most users, but then again, you don't get the offline installers if that's what you want. To be fair, if you want those then lgogdownloader is superior to any method that gets singular games in wine prefix managers or linux game launchers.
Otherwise, it's pretty similar to Lutris or Bottles. Maybe even inferior.
My use-cases are
1. Using Heroic to conveniently download and run GOG and Epic games
2. Collect my installed games in a pretty gaming library.
3. Browse all of the pretty gaming library to decide what next to play
Lutris is thus the competitor for GOG Linux gaming with Heroic as it does these things too. But with Lutris you have the option to keep the offline installers. And there are fixes to get games working out-of-the-box. Tough choice.