Lack of support. Companies don’t offer free blogs out of the goodness of their hearts. They still want to make money somehow. If you’re only using their free service, they’re not making money from you. Therefore, they don’t have a lot of incentive to keep you happy. Sometimes this means you’ll have little to no support. Other times it means you’ll be badgered with offers for their paid-for products.
Affiliate sales comprise a large chunk of revenue for most big-name bloggers. For instance, Pat Flynn made $53K last month from affiliate sales, compared to "just" $9,500 from book sales. In general, look for affiliate products with higher price points; web hosting companies are a great choice if it makes sense in your niche, with payouts of anywhere from $60-$130+ per signup.
Lets say, for example, that you refurbish and resell used smartphones out of your home. You could use a blog to attract visitors to your website where you list your current phones for sale. Your blog might cover topics about DIY refurbishing. On one level, it seems counterintuitive because you want people to buy your phones, but it also helps you build a brand and gain recognition. Social media guru Jay Baer explains the concept on Copyblogger: