My experience, so far, is that it's not that bad of a single player game.... you're just going to be spending a lot of time traveling. It's a bit like minecraft, Dark Souls, Stardew Valley and Skyrim had a child together. So, there's plenty to enjoy as a single player - but I can see how the gameplay loop would greatly benefit from extra hands. Once you get to ore processing, you really need miners exploiting an area and runners carting material back with people staying at home to keep the products coming. It's all possible to accomplish alone - just... a lot of time running back and forth across the map.

Though I have yet to really play multiplayer - so I can only guess what that looks like. So far, for me, the bosses and major battles have all been possible as a single player if you're making sure to not try and be too dedicated to one play style. I would argue archery is more universally effective than physical combat, but it does become necessary to adopt contact forms of combat for progression at least once.