Scalability is a very important aspect. I started to run affiliate sites as a hobby while playing poker professionally. Then it started to make enough to be my “job” (although I still played poker professionally) and then in about the 3rd year it made enough to give me and my husband a full time job too. I still play poker professionally and the only time my affiliate income was more than my poker income was when I was on maternity leave and didn’t play poker at all 
However, if I stopped playing now, I have a business that can make my living (not a very good living just yet, but more than most people who work in a job).