Clearly SIWA is among the walking dead

 Clearly SIWA is among the walking dead. Two out of three times that I've tried it I've been hosed. All on brand new accounts for Fing, UPS, and Yelp.

For Fing I ended up creating a traditional login because their participation was only for the iOS app, not the web or desktop version. Their SIWA setup experience was buggy and did things I didn't expect.

For Yelp I created a brand new account using SIWA (for that once or twice a year I tap a Yelp link in Apple Maps), but I haven't used it. I dislike Yelp, anyway.

For UPS MyChoice I should have stopped when I noticed the bugs during sign-up. I had the account fully set up and customized. I realized there was no trace of the password when trying to make changes that required you to sign in again, and it doesn't trigger the Safari "sign in with Apple ID password" prompt. Despite already being IN the account, UPS acted like I was 100% wrong. Digging around in Keychain and Safari Passwords doesn't reveal the password Apple had to have created, although there were two others the same minute as the sign-up that don't help.

More: https://forums.macrumors.com/threads/sign-in-with-apple-supported-apps-sites.2212193/page-31