In the Netherlands it's called water and kefir. Good stuff indeed, especially during summer and basically free at the cost tapwater. No plastic packaging either! Those hipster might actually be onto something ;)
The speed of food poisoning can be hours for some pathogens. It depends on the pathogen and the amount, and if the body detects the bad food and expels it quickly.
When I make kombucha I am quite careless in the process, but it always turns out fine.
Fungi are more dangerous. If something starts growing hair, it's out.
Dairy fermentation is more dangerous ¹, and quite hard, so I avoid that.
Alcohol is obviously a toxin too, and rather common in fermentation ;)
In nearly all cases, I'll trust one of the best fine tuned measurement devices to protect humans from toxins: my taste and nose.
(Q: how do you know the milk is beyond it's expiry date? A: Smell it! Far more reliable than a date printed on some package)
¹https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6356804/
Edit: another thing to be careful with, are kernels, pits, seeds and peels. Peels can often contain lots of sprayed on chemicals that will probably kill your fermenting bacteria. Pits and kernels can contain insane amounts of toxins that we usually won't eat because they are enclosed. But crushing/long fermenting might release them
Also watch out for methanol. Fruit pectin gets fermented into methanol so using pectase is recommended to break it down, especially if one ferments citrus juice.
If people whose day job IS testing food recipes, all end up with different results from the master recipe made by their colleague, then it indicates certain external factors need to be controlled to ensure consistency.
The factor they narrowed down to, was consistency of temperature, and the solution they offered was to suggest a temperature-controlled box to ensure best results.