The danger in the US involves general poor tea handling with iced tea. Sun Tea involves leaving water and tea bags in the sun for a long while to do kind of a cold brew. Many places (shitty donut shops, greasy spoon diners, etc) make iced tea by stuffing the filter basket on their coffee pots with tea bags, often not getting the tea up to a safe temperature to kill pathogens. It's often then let at room temperature until it's served. It doesn't need sugar in it to be a problem— heck, legionnaires disease grows in closed water circulation systems. It's not common, but in the past decade-and-a-half, there have been a few outbreaks in the US. Also, studies have shown that the containers for sliced lemons in restaurant service stations are frequently teeming with fecal coliform bacteria. If they toss lemons in there to cover up the disgusting residual old coffee flavor, even better.

Heathens over here.

So properly made kombucha wouldn't be risky, but as you noted, it isn't always properly made. You can do most culinary things wrong enough to get you sick if you really put your heart into it.