(I would personally consider doing installments at 0%, so I'm curious as to whether you were given an advantageous plan based on a credit check or whether it was the timescale.)
I only used it once out of pure curiosity, so idk all of the intricate details, but they essentially offered a 4 equal monthly payments plan with 0% interest, as long as I pay it off on time. Tested it, worked as promised, all was good.
I bet if you go outside of their deadlines for payments/try to stretch it for longer, you will get hit with massive interest rates tho, but cannot confirm that. Mostly because I only use credit for things I can easily pay off in cash at any moment (except things like car/house obv), so I basically treat it like a debit card and pay it off very pedantically on time (because with 0% interest, it is essentially just extra leverage).
The article was written in 2017, and mentions an APR of 19% for a $200 purchase via Affirm. Looking online it looks like their current rates are around 10-13% unless you pick the shortest term loan[1].
These schemes are VC-ified installment loans, which operate on the opposite model: you receive the purchase up front, and pay it down over time including interest payments.
They're arguably better on a strictly economic scale, since they give consumers the liquidity to make purchases that they'd like to make (and can actually afford, just not all at once.) They're arguably worse on a social scale, since installment loans are a form of cheap credit that people get addicted to, and are comparatively unregulated.