I strongly believe that any designer who chooses not to articulate their criticism is not one whose opinion is particularly valuable in the first place... certainly not enough to try and pry their reasoning out of them. Every product is not for everyone. There is always a subset of people's who have insightful feedback and a subset of people who don't. The UX folks at FB foreseeably are more patient but on a design forum giving a word one impression is, to me, analogous to publicly admitting that you cannot analyze products with any nuance yourself and very likely just implement other people's solutions in your day to day work.

My younger siblings have told me they use Snapcash quite a bit to pay back friends who've spotted them money which seems to be a frequent occurrence among poor middle school, high school and college students. There's this general impression that FB by itself (ie. without Instagram) has trouble keeping the interest of a younger demographic and something like this could be critical to retain them. I imagine many of the Snapcash users minus the e-strippers are, like my siblings, fairly young since anecdotally spotting people money has happened to me less and less as I've aged (friends have steady jobs, everyone has access to multiple credit cards, etc). If they're not the primary demographic at the very least they'll use it...

My siblings have coerced me to set up Snapcash and use it several times with them. My impression was that while it was very simple, the experience left a lot to be desired: it's very hard to figure out if your payments went through since they show up on the direct messages page which is ephemeral, at the time there didn't seem to by any way to view any previous transactions and their status, and there seemed to be no other ways to enable further security at all (the most you can do is require a CVV). In fact, the entire functionality seemed more than a bit tucked away ("Where the are the settings in the first place?... Oh I need to swipe down on the photo screen to get to them... I'm too old for this").

I previously used Paypal owned Venmo quite a bit which has a notoriously bad security record - actually much worse than Facebook. Venmo published a very timely post on security today and the media team, many of whom I know personally, were out performing their usual ruse of "Keep using Venmo...please?" apologetics - I'd say them being scared is a great early signal for the success for FB Pay.

More ambitiously, I think it could be the first critically important feature in building out the functionality of Messenger as a stand-alone app which obviously had a ton of pushback - the more alluring features the more it's users can swallow that separation. It's also very interesting that they released this right before F8 next week, right? I'd like to imagine they will release some sort of API connected to paying friends which would be incredibly incredibly powerful.