Great question. I stick to one broad niche per website. And I write about a bunch of sub topics within those niches. So, I probably wouldn't write about pets and the latest computers on the same blog. However, I could write about computer chips and pets on my pet blog. I definitely agree that writing a bunch of unrelated content makes it harder for a website to rank. However, I would also warn against niching down so much that you can't grow and expand your content.