As with some other mentions here, though, it was killing the ability to have Simple send checks to payees that did it for me. I'd been waiting for years to have Simple be able to accept ebills (e.g., receive a bill from a creditor that specifies an amount to pay and paying that, rather than just paying a fixed amount every period), and they never did; instead they regressed.
But here's the thing: while I'd left Bank of America because Simple's app and design was so much better, I'd never closed my BoA account for the simple (ha!) reason that they could accept ebills and pay them out of my Simple account. This always left me with the niggling feeling that while Simple was better at technology and customer service, BoA was better at... well... banking. And when I checked out BoA after getting fed up with Simple, well, their app was still ugly and had bad UX design, but the functionality had mostly caught up with Simple. It even has goals built into it, if I want them. (I don't, and never cared about Simple's equivalent, so I can't compare the functionality.)
And while there are "bank things" I only do once in a blue moon -- deposit at an ATM, get a cashier's check -- it turns out it's nice to have a bank that, well, does those things. I know there are other banks (and credit unions) that do them, too, and I'm not suggesting BoA is particularly good (or bad) in this regard.
Simple was a promising idea, but I think they made a serious mistake by letting themselves be bought out by BBVA. And while I'd love to see someone else try again, I'm going to be a lot more reticent to move to a new bank just because it gives me a terrific mobile app experience next time around. Simple was a great technology company, but at the end of the day, I just don't think they were that great a bank.