I soloed Vermintide 2 through Legendary difficulty. I consider that one of my greatest gaming accomplishments, as it's a very hard 4-player co-op game. I also did it with Guido. However, there were two or three occasions when I decided to play with randoms. In those cases, after 1 or two fights, everyone left the game, leaving me there by myself. I think they would be surprised to learn that I finished those levels without them. In at least two of these missions, the game tried to send more people to help me, and they all left the game, too.
Of course, I'm not 100 percent sure why everyone left, but it happened every time I joined randoms, so I assume I was playing differently than they played, and they decided to just leave and find someone better to play with. The only thing I noticed that was different was that they didn't hunker down and fight hordes. They kept moving. I could have eventually adapted to that, but I wasn't given time to. You certainly can't play that way solo or with one other player.
Of course, I'm not 100 percent sure why everyone left, but it happened every time I joined randoms, so I assume I was playing differently than they played, and they decided to just leave and find someone better to play with. The only thing I noticed that was different was that they didn't hunker down and fight hordes. They kept moving. I could have eventually adapted to that, but I wasn't given time to. You certainly can't play that way solo or with one other player.