Similar experience as @vknow360 with my "proof of concept" phone OTP method:
which has been available for two years. 10 likes, and only one enquiry, but I also surmise that not many have used it, possibly due to the potential security concerns with this method (which you do not mention...)
Seems most prefer federated sign ups, or plain email/password, or their own registration/login system within Firebase with no rules, although you can circumvent it to some point: Subvert FB Authentication