Filling out your 1040 is "hard" because people don't understand what the words mean, there's a 100 page instruction manual that defines the terms, and it might require filling out other forms too (which you might just need to know somehow that you need to fill them out too). Other than that, the actual UI design is straightforward. You write/type numbers into numbered boxes, top to bottom, occasionally referring back to numbers you've already completed. You could progressively enhance with automatic calculations for relevant fields, but hand calculations work as a fallback.
Reliability is unrelated from UI, except insofar as simple UIs are easy to build, and therefore less likely to break. A paper form 1040 is perfectly reliable; it's not going to burst into flames when you're filling it out. As I said above, I couldn't even fill out my payment form on a modern site. It did not work at all. The form did not appear. That is not reliable. It also makes no sense if you know the page is ultimately using HTML, and that HTML has forms built directly in, and they always work fine.
And yeah, when I'm doing something like making a payment, setting up a transfer, doing my taxes, or even ordering a pizza, something like a slightly advanced paper form (e.g. with drop downs for options) would work great on my phone or desktop. Have a special request for your pizza that's not on the form? Put it in the free-form instructions box.
The "progress" we've made in the last few years is that I can't do bank transfers without switching browsers, which requires selecting a "to" account from a drop-down, a "from" account from a drop-down, and typing an amount. I don't see how something so basic can be so hard to do correctly. There's literally no need for any javascript at all. I don't see the usability gain from whatever they're doing.