I think it depends on the exchanger site. Because I'm always leaving my altcoins in c-cex but it never lost. It would be better if you will leave your bitcoins or altcoins in some legit exchangers like c-cex and yobit to be sure that your bitcoin or altcoin would never be lost or get stolen maybe.
Is c-cex or yobit more legit than poloniex?
You have got to be joking right? Them two exchanges are scam exchanges and poloniex is the most trusted alt exchange.
I definitely agree that Poloniex is the most solid and respected altcoin exchange in the market, but that doesn't mean other exchanges are scams. Yobit has a horrible support service, which might even take much longer to reply or solve open tickets compared to other exchanges, but that doesn't make it a scam. I can't really comment on C-Cex as I simply don't have much information about this exchange, but from the looks, just as is the case with Yobit, there are a whole bunch of shit coins listed for the sole purpose of earning money. Every fake dev and its shill supporters are looking to buy themselves a position on various exchanges to dump their worthless coins on rookies.
Well i agree with your statements in the general, i just trust into poloniex at the moment its my only exchange, yobit is being used for several devs that wanna to scam, soo they play the pump and dump, the support is something they are taking care nowadays, they hired and moderator atleast, the funny thing is you have or can tip those person for he makes his work.
Bitfinex were an example about what not to do, trust all your coins into one single exchange, but since i do own a small number of bitcoins, loosing them at poloniex at the moment i doubt can happen, but if happens i know the risks i were taking.