Enemies will have more HP during multiplayer, that's about it AFAIK. In my experience the game is balanced for solo play, but fighting together opens up more interesting strategies especially at bosses. The Forsaken powers are also shared to nearby players when activated, so with that feature in mind, co-op is one cornerstone of Valheim's design and I wouldn't recommend missing it if it's up to you.

The way I see it, there's virtually no game where bringing more people to enjoy it together can be a bad thing, even if it is a single player game.

There are several fresh threads about swamp, use the search feature to find them. The game will keep getting more difficult and somewhat more complex as you progress through biomes. Walk softly.