The common denominator of the 3 websites that work well is that they are in niches that I was already heavily involved in before I discovered the world of internet marketing. I can write about them all day long and I hardly need to do any research because I already have years of experience to draw on.
The other 17 websites are frankly rubbish. They are in niches that hold no interest for me whatever. They bore me and that obviously comes over in the writing. I wouldn't buy what they are selling if I landed on them.
So if you are struggling with choosing a niche to get involved in, I would urge you to stay with a niche you know. They might not be the greatest earners. But then again if you are knowledgeable in the field it could well be more profitable for you where as for someone else it wouldn't.
I know some people will say you can outsource.
I think that real knowledge and enthusiasm comes through with an authenticity that is impossible to fake.