So if you initiate the phone-to-phone Signal account transfer process, and it almost completes, such that the account is transferred, but the data isn't, and then errors out ... and if you didn't have backups enabled ... then your entire chat history just vanishes into thin air with zero recourse. In other words, Signal nukes your history from orbit on the old phone before validating that it actually successfully transferred to the new phone.
I am indescribably angry right now.
(The extra insurance you need, that I didn't have, was to enable general backups on the old phone first, and make sure that a backup completes on the old phone before initiating the transfer.)