[lmm]

> The other difference is that I'm having a hard time imagining a scenario where someone is getting blindsided by "damn, it should've been a SPA after all!"

I've seen it happen more than once. Start with what looks like a straightforward form flow, seems like it should be fine in rails or whatever. Then it turns out some fields depend on other fields, so you've got to dynamically show/hide parts when other parts are set. And doing a server roundtrip, even AJAXey, is just too slow. So you put a bit of client-side Javascript to handle that. And then as the product evolves the interrelations between the form elements become more and more complex and businessey, and you have more and more logic split or duplicated between backend and frontend.