A few have mentioned it, but to me, the "unknown" aspects of this, which remain to be seen, are:
1) How tough will Klarna be on trade-in condition at the end of the lease? Apple's requirements for trade-ins tend to be fairly loose (especially in person). But they are Apple - the customer-facing Company. They want you to trade-in (as the vast majority are using the trade-in to subsidize a new purchase). Klarna being the counter-party here, is an unknown. There are plenty of horror stories of Apple's mail-in trade-in partner, Phobio.
Almost certainly they'll be tougher than Apple, although if enough people complain, perhaps Apple will pressure Klarna to be more receptive (or maybe they already have in their deal) for this to work and be considered favorable long-term.
2) Today, Apple's trade-in prices for 1yr old iPhones is definitely >50% of the MSRP. I don't know the exact numbers, but I think it tends to be in the 60% range (for base model - you get hosed if you upgrade storage as the trade-in value doesn't change). True FMV is even higher (eg. if you sell on Swappa or Ebay), as iPhones hold their value really well over time (I'd guess 65-70% for 1yr good condition iPhones, although there's scam risk/fees).
If the Upgrade program becomes popular, will Apple reduce their Trade-In prices to be more in-line so there isn't this arbitrage opportunity? (eg. Lease the phone for 12mos, buy-it out at 12mo period for essentially 50% price, trade-it right away for 60% and buy a new one?)