edit: I have to apologize as I didn't look at the dates. Perhaps there is still something new informative for some people to read with my post.
At least GOG for their cut are providing actual service, while Valve usually doesn't do anything at all.
Valve is investing into their client. Steamworks for devs (https://partner.steamgames.com/doc/api
). Then there is Family Sharing, Remote Play, their excellent Controller Support, Workshop, Broadcast, community features. They aren't just sitting on their money, though sometimes they are kinda slow on things. You should take a look into their Steam Labs at what they working at and pushing out.
It's each dev's respective responsibility to offer support if a game doesn't work. Valve can't be blamed for Fallout 3, for example, not working properly on Steam because it's Bethesda's duty to remove the Games for Windows Live addition through a patch. However, Valve can be blamed for not pressuring Bethesda to change it. At least people are able to voice their negative opinions through User Reviews on Steam, telling people to get the GOG version. Take a look at Borderlands 3 and Subnautica when they released on EGS. No forums, no user reviews, everybody had to go to the Steam forums to find help whenever they had problems with their games.
Valve's cut is initially 30% (like Apple, Google, Sony, Microsoft and Nintendo), for those in this thread that don't know: Since 2018 Valve's cut gets reduced to 25% after 10 Mio. $ in revenue and to 20% after 50 Mio $ in revenue. Valve also allows developers to generate free Steam Keys to sell on their websites for 0/100 split with all money going to the dev. Factorio dev does this, for example.
Another topic here is the DRM free thing. A common misconception is that, if a game releases on Steam, it's bound to DRM through Steam. That's not true. Developers decide if they want to use Steam as a DRM, requiring the client to play games. https://www.pcgamingwiki.com/wiki/The_Big_List_of_DRM-Free_Games_on_Steam
Here is a list to find some examples. There are a few differences here and there. Some games only use Steam for the first launch and are DRM free afterwards, like Cuphead.
Other games that are completely DRM free, when bought on Steam: Psychonauts, Broken Age, Wasteland 1 and 2, Europa Universalis 4, Rayman Origins, Bastion, and many others.