Exactly my experience. Heroic works fine for GOG, and unless I missed a trick, I couldn't get it to install the Epic Games Launcher program? I can login to the Epic webpage and buy games but not install as it requires the launcher. Tried downloading the Epic Launcher in Heroic and nothing happened.There are a few ways to run epic games on steam deck. Heroic Games launcher and Lutris are the most common I've seen. I didn't get on with heroic for epic stuff, though it worked great for me with my gog games. Lutris has worked great for me with Epic games so far though
Whereas switching to Lutris, it will allow you to install the Epic Launcher and games to your Deck. Just picked a random game and its installing Snow Runner as we speak.