you can for sure get some diarrhea, I wouldnt deny that. But that can happen anywhere. I think a lot of people get scared off from getting diarrhea though, when it could be the water, could be the food, could be anything. Traveling in less developed countries, I would for sure ask the locals whats up. If people live there for years and still cant drink it, sure, its fucked. If a guy went to tiajuana, ate a bunch of cheap food, drank a bunch of drinks with ice (dirty hands are bad news), and drank a glass of tapwater, then got diarrhea, there are enough reasons to doubt.
I said it before though, if you get a little stomach upset, just keep up drinking it, and if you dont have a totally weak stomach, chances are you will adjust.
I have eaten out of the dumpsters for a lot of years, drank tap water in lots of poor countries, and yeah, I get diarrhea sometimes. But fuck I drink a pot of coffee every morning, and shit water once a day anyways. Whatever, better than constipation.