One of the main problems with both probiotics and prebiotics today is that your stomach acid kills them before they get the chance to reach your upper large intestines. Your stomach acid is strong enough to dissolve razor blades, imagine what it does to weak pills! Most store brand probiotics and prebiotics aren't strong enough to make it to your gut so they have little to no effect and at times, leave you feeling worse off than when you started!
Some supplement companies want to stand out from their competition by increasing the dosage of bacteria. Sadly, bigger isn't always better as our guts are a delicate balance of bacteria. High doses of certain bacteria species can "overwhelm" the good bacteria in your gut, killing off other beneficial strains.
Once you stop taking these high dosed probiotics, more harmful bacteria can take over, leaving you with a less healthy balance than when you started.