Exchanges might be all right for coins which otherwise should be kept in specific wallets, but in general it is a bad idea to keep coins on any exchange, because the policy might change and you can end up not being able to sell coins without ID or something else. MEW supports all erc-20 tokens and since most of tokens are like that, it's a great place to hold cryptocurrencies.
If the coin has some issues it might not be the best option. For instance, I hold iota on an exchange and that it not very safe, but their wallet had been seriously hacked once, so I could lose my iotas if I kept them there. I think it is better to use a trusted wallet and perhaps even a trusted exchange if there's no wallet like that, instead of keeping coins in the wallet of a basically new project.