Coinomi looks very interesting, really holds a lot of different coins, very good for my purpose i use it for bitcoin and other altcoins , there is some altcoins like waves have a special wallet for it.
Coinomi is one of the good wallet to store the crypto currencies and it also has private keys but still you can go with hardware wallets like Ledger Nano S where the coins will be safe to longer period.Or you can use separate wallets for storing each coin will be better than multi currency wallet.