in my own experience, I am holding every altcoin's on my MyEtherWallet because for me it's totally safe as long as you are too careful to manage your wallet.
I didn't sell my altcoins so I don't worry if I lost al my assets because I had keep my private key safe and secure and I didn't saved it on any cloud storages or even on my phone, tablets and computer devices.
It's better to keep your wallet on hardware like Ledger Nano S or Trezor but I don't have one.