Reviews is not really one niche but rather a type of content that spans many different subjects.
What’s particularly interesting about reviews though is just how many niches get covered. For example, you can actually grow a very large audience by reviewing toys of various kinds! If you remember Transformers toys from your youth, then try typing in ‘Optimus Prime review’ and you’ll find there are plenty of videos of guys playing with their toys, transforming them and narrating the experience… who knew!
Fails are compilation videos of people falling over, crashing vehicles and generally making fools of themselves. If you can find fail videos that have no copyright, then you can create your own collages with no need to go in front of the camera yourself. ‘Wins’ are also very popular!