When I set up a client sftp drop I had the luxury of saying "key based access only"

Then it was a surprisingly customer service intensive process to get the keys from the customers. I never did manage to automate it fully. Weird enterprise systems I have never heard of with strange key formats. ssh1 keys, email mangling them. Their internal processes requiring a password.

And way to many "I am sorry but that was the private key you sent me, That key is now compromised and you will need to remove it and generate new keys anywhere it was used"

I almost regret not allowing passwords, almost, despite the headaches key based is still much better.