As I understand it, people were mad at Epic when many game projects on Kickstarter, formerly announced and crowd-sponsored as multi-platform, were suddenly funded by Epic and changed to be exclusives on Epic. Many people viewed this as a breach of trust by the projects, and as even worse done by Epic, by prompting and encouraging this.

I think more competition in this space is great, and overall we'll all benefit. However, notice that when you open up Steam, the default view is your library of games, whereas when you open the Epic Games Launcher, your default view is the store. This suggests to me that BD folks are running the show, rather than folks who are interested in games. Valve has spent the better part of the last 20 years proving that they generally make decisions for Steam that put gamers first- enough so to convince me, a dedicated FOSS and free culture supporter to pay for DRMed games. I've also purchased a game from the Epic Game Store, and while the experience wasn't as good (the Epic Games Launcher is pretty half-baked software), I expect them to iterate and improve it, unlike EA's Origin software which is almost comically broken.

There has been controversy around them paying third party developers to release their game exclusively through Epic Games Store. From what I recall these are timed exclusives so the games do eventually end up on other store fronts but people seem to be really up in arms about it.

Some of the complaints are about Epic's lack of any interest in the minefields of Linux and macOS support. It's not a PC gaming store, it's emphatically a Windows gaming store. (Some of those complainants seem to overlook how long Steam was Windows-only or how much of the Steam catalog will likely remain Windows-only.)

Most of the rest of the arguments seem to revolve around Epic doing paid (timed) exclusive deals with an interesting cross-section of developers. (Using the weight of their Unreal engine licensing fees to sweeten the pot of what they could offer in some cases.) It's something common to console games, but some PC gamers seemed disturbed to see it on the PC. It also seems to betray a short-term memory with respect to Steam, as Steam also bootstrapped on exclusives such as Half-Life 2. (Half-Life 2 wasn't even a timed exclusive as most of Epic's have been, HL2 still requires Steam in 2020, whether or not you bought it or The Orange Box on a physical disc from a retail store back in the early oughts. Which leaves prevarication room for some Steam fans because Valve did allow multiple stores to sell the game even if it was a platform exclusive to play the game; but it still ignores the historic controversy when Steam did that in the first place and many gamers at the time called it a mistake and the death of PC gaming.)

(ETA: Also, Epic did do one sleazy/evil thing early in the EGS: it scraped private Steam files for friends lists, rather than using documented APIs, to try to bootstrap its social network.)