Too much doomsday ITT - but then again, with few exceptions, I don't buy modern games on GOG, only the classics.
My GOG account is almost 14 years old and across these many years I honestly haven't had much to complain. I get exactly what I want from it, a place to have my classic PC games library for reasonably cheap prices on sales. I think Galaxy is pretty serviceable, the library looks very good over there (so much so Steam now copies its look), the download speed is alright, I haven't had any technical problems. I don't get what's so awful about it, I assume maybe people downloaded it way back in the early versions and it must've sucked back then (during the time I was using offline installers exclusively), but now I can't see the issue.
Both Steam and GOG have their strengths and weaknesses, the ideal scenario is to have an account in both and play to each's strengths. You might not like it but it's definitely not optimal to have only one of them, that's the harsh truth. Steam is the safer ecosystem: you know for a fact with 100% certainty that Steam will still be around in 10 years, but the same can't be said about GOG unfortunately. Steam also tends to always be more up-to-date and receive updates/patches faster - it's the dominant PC store after all, devs might prioritize it, especially when they look at graphs saying 90% of the game's PC playerbase is on Steam, 9% on GOG and 1% elsewhere. On the other hand, do you reeeeally own your purchases on Steam or are you permanently at the risk of losing your entire library if some purple haired Seattle landwhale dislikes your profile/forum posts/etc? Under Gabe, Valve seems pretty chill when it comes to these things, but company cultures change over time (just look at Blizzard) and he won't live forever, especially with that body.
GOG on the other hand allows you to download the offline backups, so even if Treespirit Kowalski xe/xhem decides to fuck you up you can still have your library. The classics library is far more complete and I find them to be generally far easier to run on W10 without hassles than the Steam versions. They also provide AAA/modern games because they don't want to be forever trapped on the classics corner. They don't want to restrict themselves to be only a digital antique dealer, and I understand that decision. But... You know what they say about not ordering steak on a seafood restaurant?