I personally don't have a Ledger Nano but would really like to have one. People say so because the ledger nano is considered as the safest among any of the options available. Exchanges on the other hand are not considered safe because it is their owners who are handling our accounts and it is them who are owning the private keys of our wallets. Anyone who owns the private key owns the coins. So it is them who are holding our coins whereas we own the private keys ourselves in the ledger nano.
These are all valid points. However, please consider that there are exchanges or online wallets that allow you to be the owner of your private keys. In such a case you can use your hardware wallet to store the keys for an online wallet. As the keys never leave the device, it is perfectly safe and secure
Also, if the exchange you choose doesn't allow you to store your private keys on a hardware wallet, you can still use a hardware wallet as a 2FA device. It is considered MUCH safer than using a phone as 2FA. You see, it is extremely easy to hack a phone and extremely hard to hack a hardware wallet.
So this way or that way - a hardware wallet is the way to go