I think, single niche blog is better than multi niche blog. But I can’t say that multi niche blog is bad. I think, for a technology interested author, Multi blogging is best. As he can write about Windows, Linux, Android and everything he wanna write. But as you mentioned, SEO of multi-niche blog will be tough. Actually, If the author of that blog have much experience of SEO, then it would not be so tough and the second one, Making money from your blog will be tough. Yeah! this is true, but many bloggers done it and got success and the third one is not 100% true. Getting readers in Multi Niche blog is not tough work if you make you blog categorized. Even, You will get visitors from many niches which is good and many people love multi niche blog. Let’s take an example of the blog ‘Hacking University’. This blog is making money and is a multi niche blogs. It is a multi niche blog.