not always, looks at bitstamp and bitfinex they are famous exchange and they were trustable back then but they got hacked, no exchange should be trusted they are subordinated to hackers
good services maybe, but trusted no, my point is that they are being used only because they are the most trusted, but no trusted 100%, and you are in the end to be bound to use them, you have no choice, they are the less worse out there
if they are 100% trusted then why no one leaves more than what you needs to do trading in those exchange? because nothing is 100% trusted, at best it is the most trusted
usually big traders, or everyone with some intelligence, don't hold all his money on exchange, but only those few coins that he need there for trading, otherwise you can't catch the P%D to perform your move quickly enough in the market
i mean that if you do not want that hacker steal your coins , you do not leaves them on any exchange, minus thsoe used for trading, because every exchange has carry a certain risk, and those big trusted exchange are not excluded