All Exchanges are not created equal and one can still lose exchange coins due to exchange getting hacked or shutdown due to government regulation or some other reason. So it is useful to keep a good portion of your coins in your own wallet. To reduce the risk, use multiple exchanges and multiple wallets too.
Actually, both wallet and exchanges is not a hundred percent safe. Your wallet can be hacked, same with exchanges. The difference is, most wallet are hacked because of owners negligence, and with the exchanges, it is more of lack of security of the platform itself. If you are holding for long term, it is better to have a hard wallet to store your coins.
Thank you