For starters: complete lack of features and user support. EGS gives you the game and basically no way to interact with the community around that game. They don't support Linux, which is huge for some people, but also makes some peripherals like Steamdeck that operate on Linux entirely incompatible.

Because their user support is so bad, nobody really chooses EGS to buy/play games from, so Epic tries to take that choice away buy giving payouts to publishers to only let the game be on their store for six months or longer, meaning anyone who wants to play such a game has to come to them. This is also why you see a lot of free games, EGS trying to lure people to their "service".

Which is where the real big problem comes in. Instead of user beneficial features, most of the storefront and game launcher is bloat ware that would rather show you more and more ads for other products on their store than let you get into the game you want to play. And if reports are true, advertising games already in your library. So they aren't even trying to tailor a custom user experience, they are just blasting you with a bunch of shit till something sticks (or you uninstall)

There have also been allegations of EGS scanning personal computer files outside of its install directory, which is scummy enough on its own but its also transmitting that data back to their central server, which gets handed off to Tencent, the Chinese owned company that is a big investor in Epic and has their own history of scandal and anti consumer behavior. So if this all is happening, its hard to say just what data on your computer is behind handed off to Tencent and the Chinise Government because you wanted to play a silly game on an inferior game service.