MEW itself is safe in the owner's responsibility. Infact, MEW is developed for full control and right of the owner over his wallet. There have been cases of MEW being hacked, however, only because the wallet owner was not reasonably careful enough to let hacker either have the private key or install harmful software in to victim's computer. Thus, we strongly recommend all crypto-investors (not just ours only) to store your key information in a "cold wallet" beside what you have secured in the network - the "hot wallet"