Your MEW wallet will be safe until your private key is safe so it is better to understand that myether wallet can't be hacked and it is better to keep your system offline so no phishing links can steal your private keys.
Even though MEW is only for ethereum and tokens but it doesn't support other popular altcoins like litecoin, bitcoin cash or monero.So Coinomi is one of the best wallet to store multiple coins and it is nkt a better idea to store yiur altcoins in an online exchange.