In a security system there must be a gap that is very guarded by the system. all depends on the security of the website and the people behind of the website. there is Many reputable exchange cases have to ended horribly and claimed the company was hacked and the coin was lost. if we remember the past like mtgox and other exchange that are trusted exchange at that time. but the possibility of vulnerability of stil systems exist these days. so this why i just believe with my own wallet . im just using exchange for Exchange to real money, not as my wallet to saving money there.