Too complicated

There was a girl who only recorded the private key of a certain mobile wallet address and did not record the mnemonic phrase. After she sent coins once, the coins were transferred to another address. She forgot the unlock password for her iPhone, which led to it being permanently locked. When she took it to the U.S. for repair, it became unbootable, making recovery exceedingly difficult, and she could only hope to recover it through the phone.
Most wallets have a change mechanism for token transfers in the Bitcoin system, meaning after sending coins once, the remaining coins will be sent to a new address (this address is called the change address). The private key of this new address is generated by the mnemonic phrase, so simply recording the private key of one address is insufficient, resulting in the private key of the new address not being recorded.
Therefore, wallets with a mnemonic phrase must record the mnemonic phrase.
Similar to Bitcoin Core wallets, you must also save wallet.dat, not just record a single private key, because this file contains all private keys.
(1) If you only record one private key, after sending coins once, the address corresponding to this private key will have no coins left.
(2) Whenever the number of addresses you use exceeds 100, you must back up wallet.dat again.
This is because the addresses in wallet.dat are generated in units of 100, and the first 100 are generated at the initial run. When you use the 100th address, the wallet will generate the next 100 addresses.
If you use the last 100 addresses without backing up, the private keys of those last 100 addresses will be lost, along with the coins. This situation often occurs in mining wallets.