That all depends on the users. Carelessness is the only thing that will lead for their wallet to get hacked. There are hundred of thousands of users and if it isn't safe then MEW would not still be operating at this time and they are totally out of the system.
myetherwallet is really one of the best wallet used by the people and the crowd of people on that wallet means that people are feeling safe on that wallet. Suppose if there were any issue to people or any risk for the users of myetherwallet then there would have many complaints against that and people left would be very less. But in real people like that..