Outriders play well solo, but you'll want to pay attention to your world level.
I'll take that statement off right off the bat: Outriders play better with a team, as the four classes synergize pretty well together. That said, single player play isn't inferior at all. You'll miss out on some awesome cross-class combos with your skills, talents, and mods, but Outriders are by no means a terrible single-player experience.
Having played the demo throughout both in a group and alone, I found the solo to work very well, but with one major consideration: World Tiers. World Tiers are how Outriders handles difficulty: each level increases the challenge, but also increases the available rewards. By default, the game will automatically increase your world level (you level it up, as shown by the yellow bar below the blue experience bar for your traditional level). Sometimes you'll find yourself in a new world level without the gear you need to survive, and this is where playing solo in Outriders can get a bit tricky.
Fortunately, you can adjust your world level at any time by opening the main menu, where you will find that world level is the first option. You won't have to reload your game or restart whatever encounter you find yourself in - the world level can be adjusted on the fly. It's great for those times when you barely find yourself in the junkyard, as you can instantly lower the world level in Outriders until you get that shiny new drop that will keep you rolling back.
World level autoscaling can be turned off in the game setup menu and in the world level menu itself, which I recommend you do if you plan to play Outriders solo. You can adjust the world level in Outriders quite easily, and doing this manually will ensure that the game never becomes more of a challenge than you can handle: perfect for single player players looking to have a good time without having to train other people. .