Is it safe? Does it have advantages or disadvantages?
it is only as safe as trusting a third party can be safe!
they can run away and scam you, they can get hacked or say they were hacked and you lose money, your account can get hacked, and a lot of different ways to lose your account and with it your bitcoins.
the only advantage that i can think of is for traders who want to have their coins ready to sell and then buy back just as fast.