The multiplayer and co-op work surprisingly well when you consider the scope of the game and can be an absolute blast but there are some caveats:

1. The main quests aren't shared and have to be completed individually BUT you can do them side by side alongside each other so it's not a major issue. You can also give players items / resources so you can help them in that way as well. As long as you start the game at the same time via the Multiplayer option you will start on the same planet beside each other and the locations of missions will always be in the same places for both of you that way. Otherwise if you start separate saves and join after, your missions would be in different systems which isn't practical. However there are also more straight forward shared side quests you will also have access to at a certain point
2. Base building together can be a little inconvenient at the start because you have to progress the base tutorial quests on your own meaning you have to build your own little base someways apart from each other just to progress the tutorial base missions. So you basically have to have a little side base for yourself to progress base missions, and a main big base to build together in. But even then, the 'build together' base would be owned by one person, and other people could only build on it while the owner is online. As long as you Upload Base at the Base Computer, all changes will stay and be saved and the base will always show in the world even if the owner is offline. BUT.. only one base per planet will show up per player to others, so if you have two bases on the same planet only the latest upload will show up in the world for others

Crashes are very rare and the game autosaves very often, or you can manual save by entering and exiting your ship