We have tested several ozone generators from China, for example, and have found that they produce much higher levels of ozone and for much longer time periods. This means that they do not conform to the guidelines set out by the Health & Safety Executive here in the UK and would be considered dangerous to health.
Most of the ozone generators we bought and tested came with no instructions or a very poorly translated and hard to understand manual.
These products are created specifically for cheap export and whilst they might follow certain factory guidelines, the ozone generators are created in such big volumes that very few will have been tested. Any requirements imposed in other countries for the manufacture of ozone products may not be the same in the UK or exist at all.
You can expect a far shorter product life from these cheap ozone generators, as they will be made from low grade materials.
Used correctly ozone is entirely safe. But please make sure you do not risk your health by using cheap unregulated equipment.