The big problem with the pr*biotic market is the huge hype and overwhelmingly high amount of non standard Amazon reviews of products (not verified purchases, 5/5 ratings, people posting a first review or just nonsense). Not to mention the false advertising. Basically its gotten to the state of the diet pills.
I have seen some products that are backed by studies, but after closer inspection those studies tend to be of very low quality and heavily funded by the companies (in one study they actually wrote that at the time the scientists were not working for the company, but now they are... however there are no conflicts of interest to report...).