Assuming that quality control is good (which I think is a valid assumption), store-bought kombucha is safer than homebrewn kombucha. That’s because, quite simply, store-bought kombucha contains, well, kombucha. Homebrewn kombucha is all too easily contaminated with bad fungi or other micro-organisms. Luckily, our stomach acid and our immune system deals with these contaminants most of the time, but it may be stressing your system nonetheless.
The sugar that’s listed as an ingredient is the sugar that’s used to feed the kombucha. The kombucha needs sugar to grow and prosper. It converts this sugar into alcohol, so yes, alcohol is a necessary ingredient too – unless it is removed afterward in a refinement process.