Not safe at all. Every major crypto exchange, even those with volume reaching hundreds of millions of $$$, has been hacked in the past and there is no guarantee that it won't happen again in the future. Plus there is also a chance that a government could seize your account. Any legal exchanges are very likely to comply with any government order. That said, the best way to store your cryptocurrency is in your own wallet. You can guarantee your own coin safety. You handle your own coin and nobody else have access to them.