As some may notice, I don't typically publish posts that are too similar too close to each other. Not that there's anything wrong with doing so, but since I enjoy writing about a variety of things within my niches, I always space the posts out. I move things around on my editorial calendar a lot and it's almost always for this reason. If I think of a new post idea, I fit it in where ever there's a space of time without that topic.
Figure out what's working best for you
There's a fairly good chance that one or two of your blog post categories will do better than others. Whether this means that you stop blogging about the category that doesn't do as well or keep it going because it's something you love, depends on your wants for your blog. I definitely have seen certain posts do much better than others - and I think that's also where it's important to bring in one or two posts per week that are "Pinterest worthy" if that makes sense.
More monetization opportunities
Again, I'm sure there are a lot of people who would agree to disagree, but being a multi-niche blogger can lead to more campaigns and opportunities. Networks like Social Fabric and Clever Girls have a variety of campaign opportunities for various niches - if you're opened to blogging about more than one thing, you can therefore apply to more opps. Side note about opportunities: I don't believe that you should take on any campaign if it doesn't truly fit your blog or isn't something that you totally believe it and genuinely want to share with your readers. My point is, there's tons of campaigns that offer a huge compensation fee but if it doesn't fit your niche(s), it most definitely isn't worth it. I apply to campaigns very selectively and accept collaborations, etc. selectively too. I think this is the most important way to stay genuine - especially if you're a multi-niche blogger and are applying to a variety of campaign topics.
SEO is your best friend
This is a given for all bloggers, but if you are running a multi-niche blog, it's so incredibly important to be familiar with and totally rock SEO! Social media is great, but to generate serious traffic for ALL of your blog topics, SEO is key and something you should definitely be focusing on. When I first started blogging (and even up until maybe the early fall of last year...) I didn't focus on SEO nearly as much as I should have. As I grow as an SEO-using-blogger, I learn more and more and I've been jotting these things down to share just as soon as I feel like I have some credibility in that field ; ) optimizing your keywords is super important for all of your niches and the sooner you take on the task of getting there, the better!
Many niches should fit like a puzzle
As a multi niche blogger, it's important to stay true to yourself and what you believe in - but there are limits on being a multi-niche blogger. It's one thing to run a blog with a few niches; fashion, beauty and lifestyle or travel, food and lifestyle. But if you're going to try and run a blog with categories like fashion, politics and DIY, you're likely going to have a really hard time. I think this is a given, and it may sound like a contradiction - but my point is that blogging about multiple categories should have its limits and not be totally drastic.
Never ending blog post possibilities
Ahhh - the best part!! When I began my blog, I always thought I would run out of post ideas within a months time. Granted you'll likely never run out of post ideas even if you are a one-topic blogger, but if you blog about a couple different things, you can easily switch it up to 1.) never get bored and 2.) never run out of blog posts ideas, like ever.
Higher readership rates
If you're good at what you do and are influential and stick to your guns, of COURSE you're going to be successful! There's room for success for everyone in the blogging world. This brings up the point that - if you're blogging about travel and food, you're then rallying in those who are interested in travel and those interested in food/recipes. See where I'm going with this? If you're blogging about one category, your main focus is to reach all potential readers interested in your category; which is great if you're focusing on one thing and can drive as many people in as possible. But as a multi-niche blogger, I find that doing the little things - like being a part of multiple Pinterest group boards to pin different posts to - helps tremendously in gaining readership. If a reader likes what they see, they're also more likely to click around and read into other posts too.