Multiplayer is the main issue, if it was cross platform great, but that happens rarely

PC people will play with PC people regardless of if they got the game on GoG, Steam or Epic Store. The Console people will play with Console. So you shouldn't have too much too worry about.

I think it is worth buying at GOG. For DRM reasons it is my primary store front. Crossplay is becoming more and more common and there will probably be lots of people buying it at GOG so random match making should not be an issue (if it supports that). Maybe it will allow you to host via LAN for co op and MP as well.

Last I heard the co op/MP only supports up to 3 or 4 players per map I think.

Though, TBH, I never look at MP aspects of games much when buying them. To me co op and MP skirmish kind of gameplay is an added bonus for those that like it. Those add some longevity especially if custom maps can be made. But, I am in it for the single player game.

WL 3 co op has me interested though as it is an optional part of the main game and can be progressed regardless of the other person playing. But they can drop in later on and see how things have changed.