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.
Once you have a popular blog, advertisers will be hounding you for the opportunity to advertise. The best way to take advantage of this situation is to use Google Adsense. They find the advertisers for you and all you have to do is place the Google Adsense code on your blog to start running ads. Google Adsense takes all of the hard work out of the process and just cuts you a check.
Working with advertising networks isn’t your only option when it comes to selling ads. If you end up with enough traffic, advertisers may come directly to you and ask you to place their ad on your site. You can also contact advertisers yourself. The biggest difference from the above mentioned option is that there is no middle man, which means you can set your own ad rates.