Played a lot Solo and Coop;
- you can do everything solo, its not that hard if you learn the basic movements
- for a "survival game" its not hard, there is no thurst/bleeding/sickness/need for sleep etc.
- the resource gathering happens faster if you play with more people
- exploring/fighting in a group can make things even too easy if you play with a lot of skill
(depends on the group)
-> my advise would be an relaxing/enjoying playstyle solo or coop, just dont rush things since valheim is no rushing game in my opinion