Exchanges are not at all secure. We must avoid keeping money in exchanges, whether it is centralised or decentralized. These resemble a market where you can only transact. You run a very high danger of having your money or cryptocurrency stolen. So, absolutely, use them only as necessary. Additionally, as is always recommended, keep your money in offline or hardware wallets. If you don't, nobody will be able to stop you from losing your hard-earned money.
saving assets on an exchange, that's a pretty stupid thing, I had a bad experience where in the end my funds couldn't be withdrawn because I kept my crypto assets on FTX, when their exchange collapsed they would automatically freeze all the assets on their exchange. 99% safer to store assets in a personal wallet.
There's no way to verify centralized exchange! they're not open source, not giving your seed phrase, etc. If you use centralized exchange, it's mean you're trusting your money to them and you need to accept all the risk when the exchange bankrupt, confiscate your funds, terminate your account etc and you don't have any way to recover your coins. So it's mean there's no such safe in centralized exchange!
when we send assets to CEX then they are no longer our assets because they are the ones who already hold the assets, so I usually use exchanges only to exchange my assets to my local fiat currency and vice versa, but to keep crypto assets for the long term, I really avoid storing it in CEX or DEX.