Target Your Audience With Your Reviews
Whatever your niche, there are related products or services that your readers buy and use. Those should be the subjects of your reviews.
You want to present your readers with the things they might actually buy. Reviews unrelated to your niche are not just a waste of your time, they could even lose readers for you.
To use wildflower photography as an example niche, you could review cameras, lenses, accessories, places to photograph flowers (work in products by mentioning places to stay, appropriate clothing, cameras and lenses needed for the location, etc.), print services…the list is long.
However, those same visitors don't want to see reviews of stereo equipment, children's toys, cooking utensils and so on. They came to your website to find information about wildflower photography. Don't disappoint them!
As another example, think about a website devoted to a subset of knitting. You can review yarn, knitting needles, pattern suppliers, books, knitting bag. You get the idea. Just keep the reviews related to the niche.