"FPS has a thousand error states, and payments fail all the time with little to no explanation."
I am live on FPS and have seen zero failed transactions apart from those in testing I intended to see fail. By chance was your transaction amount in the form $x.7x or $x.8x? If you were in the sandbox putting a 7 or 8 in the tenths place is the documented way to guarantee a failed transaction and receive a generic error.
"Good luck with the resolution process, too."
Did you search the developer forum? If you did and couldn't resolve your issue, did you post your scenario and relevant code? The community and AWS staff are usually very helpful if you are polite and can describe your problem clearly.
As for this new PayPal API, I just read the developer doc and although it's missing a lot of information PayPal is obviously cooking up a service that may rival FPS. Regarding your point about massive complexity I agree the documentation can be a little confusing. Once we see PayPal's full version we'll be able to compare them side-by-side but one thing is for sure, neither was intended to be plug-and-play.
What I'm concerned about with Adaptive Payments/FPS/Checkout/etc is the friction point introduced by the co-branded UI. Each of these payment services requires you to send your customer away to log into PayPal/Amazon/Google and stay there for a couple of screens. As developers the big question is whether the programmatic features outweigh the conversion loss from defecting customers who feel the experience is unprofessional.