First rules of crypto: NEVER LEAVE YOUR FUNDS ON EXCHANGES OVERNIGHT. USE EXCHANGES ONLY ON THE MOMENT OF TRADING NEEDS. Or, at least, do not store more funds than you can afford to lose on random exchanges.
If you do not own the keys, you do not own the coins.
Well, but if I am a trader, what should I do next? Just everybody keeps funds on exchanges, but I prefer bigger exchanges with good security like binance, okex.
And yeah, if you have big portfolio, then I would split it between cold wallets and several exchanges!
when you put your funds on third party wallet, of which you do not own the private key nor direct control on it, you accept the risk that your coins can get lost.
If you are a daily trader my suggestion is to deposit coins when you start trading and withdrawal at the end of each trading session. If the fees are higher than your profits, maybe you even can stop doing such movements.
If you are a medium-long term investors then it feels natural to me to withdrawal coins as soon as I bought on marketplace.