Not sure if it's the safest way to generate a private key, but it's definetly the most fun way to do something like that.
If you generate 256 bit of your private key flipping a coin, you can 100% be sure that all your 256 bits are absolutely random.
If you generate a private key with the help of some software/wallet/other service, you can not be sure in their random algorythm. In some cases they also could be pseudorandom, generating the predefined keys within the predefined range (even if you use them offline).
I don't agree that the results are absolutely random. The coin weight might be a little biased, and you may have some muscular memory also which may lead to less randomness. Ofc this it is enough random for a private key generationg...... But I believe that are some better computer-based methods.
I still think the most secure way of generating a private key is through a hardware wallet. However, for educational purposes, this flipping method is amazing.