I was using game maker 8, and with working solo its great because you have a lot of creative freedom, don't have to worry about disputes or coordinating a team of people to help you develop a game. Working solo ends up becoming hard with bigger games as you may need skills that you aren't to well versed in which can cause a decrease in quality or because you haven't got anyone to bounce ideas off of you aren't able to develop the plot and discuss mechanics. I would say the hardest part is if you lack a skill that would be pivotal in creating a well rounded game. but if using a simple to use engine and practicing your own skills on small games working solo is fine and really fun.