I've done my long trip solo and I definetly don't regret it. Though I had started solo it doesn't mean I spent all the time by myself. I found it very easy to meet fellow travellers/cyclists on the way and to join or to make friends for a while. If you are interested in meeting locals on the way, travelling solo is a big advantage for sure. Sometimes you might find it hard to cope with all problems by yourself but on the other hand you are always free with your decisions. If I'd go in a group for a longer journey once, I would make sure to be able to split up for a while. There's nothing worse than being forced to stick together all the time and on every step you take.