Is it safe to use a non-isolated buck converter for an LED driver?
Arshid.

The simple answer is no. It's not safe.

That is, it's no more or less safe than any lamp which runs off mains and is not isolated. If you pay close attention to design and construction, it can be OK.

As for longevity, that depends on the design, the parts used, and the construction techniques. So it's not as if I can even try to answer that part of the question. However, since you missed the first part, I'm not optimistic about this part.

As for the last, a mains-powered LED driver is, or ought to be, CC by its nature, and by its nature simply doesn't care about PFC.

And let me be blunt. Please, Arshid, stay out of the LED lamp business, at least until you have learned a LOT more about electronics. Right now your level of ignorance, when applied to this forum, is obvious. When applied to the idea of designing, manufacturing and selling a commercial product, your level of ignorance is frightening.