According to the World Health Organization (WHO), probiotics are “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host.” The main issue arises from the fact that ingesting billions of live organisms doesn't inherently mean you'll be receiving any health benefits, and this isn't something that you'll learn from browsing the aisles of a health food shop.
However, the director of the Microbiome Institute, Jack Gilbert, highlights the most troubling issue through his article in 'Nautilus: a Dearth of Research.' “The lack of clinical trials demonstrating efficacy is the major obstacle to knowing whether [probiotics] are better than existing formulations that have been shown clinically to help with diarrhea and atopy. Claims that probiotic mixtures will help to support overall health are still woefully unproven.”
Additionally, due to the fact that the US industry operates with very few tangible regulations, it's almost impossible to be 100% sure of what you're putting inside your body. A number of studies have shown that there are regular discrepancies between the contents of a product and the ingredients mentioned on the outer label. What's more, this problem is particularly prevalent when it comes to products containing multiple bacterial strains. For example, a 2015 analysis of 16 probiotic products found that the contents of only one of them perfectly matched what was written on the label.
Despite the fact that a large proportion of commercially-available probiotics make unfounded health claims, this doesn't mean that their health benefits are non-existent. There are many different strains of probiotics and they each act in unique ways. Unfortunately though, only a very limited amount have been proven to be effective through clinical trials, which means that many people are buying products to reap benefits which they never receive.
Let's take yogurt as an example, which typically contains two dominant strains of bacteria, namely Lactobacillus bulgaricus and Streptococcus thermophilus. The problem here is that neither of these is resilient enough to get through the acidic gastrointestinal tract alive, rendering their benefits essentially useless. However, certain types of yogurts are enriched with different strains, which are able to get through the gastrointestinal tract in one piece. A couple of examples are Yakult, which contains live Lactobacilli, and Activia, which contains live Bifidobacteria, both of which have been concluded to be of benefit to us.
In the meantime, researchers have been uncovering connections between serious health issues, like autism and Parkinson's disease, and the presence of gut bacteria. They're also looking into the potential impact that gut bacteria have on the efficacy of medication, and are looking for ways to use probiotics to tailor-make cures for a whole range of diseases, as well as to extend the lifespan of human beings.
However, for the time being, we should all temper our optimism with a healthy dose of scientific skepticism. Probiotics are certainly not a 'cure-all' and you'd do better to opt for products designed to target a specific area, as opposed to those that claim to promote more general wellbeing.