Biggest drawback with folding bikes is that they require more diligent maintenance, yet are often chosen by people who are quite.... casual riders - to use a polite term

tiny wheels with skinny tires need daily topping off lest you pinch flat or otherwise destroy sidewalls
wierd cable routing prone to go out of adjustment and needing often lubrication
folding hinges and quick releases which need careful tensioning, and will inevitably loosen up and die of metal fatigue...
inherently weak frame designs
etc