Having a blog and putting some ads on it can really generate income specially if you had lots of unique visitors. But the only problem I saw in having a blog is the maintenance of it. Maintaining a blog was not that easy. You must really have a good writing skills and a good topics to be discussed there to attract viewers and readers.
Actually you can write anything and everything that you want in your blog just be sure you will not be sorry for what you have written cause all the people who used internet can easily access your blog and read everything you put there.
So before you put anything be sure you know your limitations.
It can get difficult when you run out of things to say, too.