I have been playing Co-op with my friend recently and it is pretty much unplayable. The hosting player will lag and stutter constantly.
But the real issue is the connecting player. The connected player has build glitches where the built item will not show on screen for that player but the game says it is there but cant build apon it. The host player is able to see it but I have to delete it and rebuild it before the connected player can see it. The connected player has issues with pulling up the scanner, build device, deconstruct device and such. The connected player can not drive any vehicles, including trains now, due to it constantly glitching around. The trains will tell the connected player there is no power but it still moves jumping and glitching all over the place. We set up stations and track and then went to drive the train and neither one of us can without it jumping 10 feet down the track due to sync issues. If a player dies while in co-op the body of the player just stands there in the "T" pose and nothing can be done. Also on our co-op save if I load the game, even if the other player isnt connected, it shows that player model just standing there. I have tried most of fixes i have seen like turning off "send data" adjusting the network speed setting, trying hardwire internet over wifi. But no solution has been found. We even uninstalled and reinstalled the game. We play co-op with other games and never have this issue so I'm pretty sure its not our networks or laptops. We have quite the speedy internet speeds at our houses so it shouldn't be that.
Hopefully there is an answer that works for the time being until it is fixed fully down the road.....we love this game to the max and it completely limits the co-op enjoyment that this game is capable of during this early access period. I see youtubers with flawless coop connections but they haven't encountered issues. Wondering whats different about their connections.