Do you mean the pre-build niche websites?
They are often crap, because if it would be very easy to earn good money with them, the providers would keep them for themselves. You usually get some max average quality content together with a low search volume search term that might be low competition.
I would advise you to try to learn how to create Niche sites yourself, how to do your niche and keyword research and how to do SEO. In the beginning especially how to create a website that has the nearly perfect onpage SEO. After you build your first website and tried to optimize everything, you can slowly start off link building. There are some very good guides out here. It's actually no big thing if your first website is shit and you fail. That's how life works. If you keep on it and give it some time, success will come for sure. SEO isn't anything like a get rich quick scheme, but if you take your time to learn and keep on it, you are able to build long lasting and passive businesses.
If you really want to go into it, here are a few threads I highly recommend:
All these members did mostly wrote at least 1-2 more awesome guides, just check out their profiles. I'm actually really sorry for not linking out to the awesome content created by other members here, but I just wanted to point the OP in a direction. If you do some further research on this forum, you'll find much more really good reads that will point you in the right direction.
If you take it serious, you'll start reading through all of this and you won't need to buy some of those pre-build niche sites.
