Thus far, anything can happen. MT. GOx was the best crypto exchange and the largest. People thought it was legit and it really was... but all it took was one (ok many) insider hacks... and suddenly it went down without allowing too many withdrawals.

I personally think Kraken, Bittrex, and Binance and Coinbase are the safest bets. Poloniex has a lot of FUD, and I'm not really sure if anyone uses it anymore when all the new coins appear on either rex or nance. And Poloniex often has server issues that screws everyone over.

But i really feel like it just takes one really good hack to take down Bittrex and Binance.

Coinbase can get hacked, but it has backing by the federal reserve, so I think it will survive. Kraken seems like a solid exchange too. Kraken was around since Mt. Gox, and has remaind strong without any hacks.