...What if the manufacturer only provides a certain range of words to generate a weak private key, which can be gained by exhaustive attack method? After all, you can only comply with the seed words it provides.
That isn't strictly true... you don't have to use a seed generated by the device itself.
You can restore any BIP39/BIP44 compatible seed into a Ledger Nano S... so you are free to use another tool to generate a 24 word seed. Theoretically, you could even generate it offline using dice and some maths and then "restore" that seed to the Nano.