Originally posted by Cooperal:

Statistically all enemies are tougher as Dialtone said. In practice however most enemies are easier as one or more people can continuously land free hits so long as they are not the target of aggro. Especially during stagger/parry windows this can easily make up for the enemy health bonuses.

This is true, usually multiplayer is easier, as long as your playmates know which is the business end of a bow. Boss fights would benefit from some kind of mini boss helpers when there's many vikings, something to even the odds besides just bonuses.

It depends.

Boss health is set when you summon them and gets set by number of people in the boss "zone".

All creatures gain 4% damage and 40% extra health per extra person in the area.

https://valheim.fandom.com/wiki/Creatures

Statistically all enemies are tougher as Dialtone said. In practice however most enemies are easier as one or more people can continuously land free hits so long as they are not the target of aggro. Especially during stagger/parry windows this can easily make up for the enemy health bonuses.