I said this on a thread complaining about SPAs a little bit ago, but I love that the SPA trend has caused all kinds of web apps to open up APIs to their users. It's not as fun as pure screen scraping, but it is very exciting when you figure out whatever weird behavior they're expecting, and it starts working.

If you get stuck, look at their javascript, see what it is doing. double check your network requests in developer tools, some of them might be more important than you think, plus it's so nice that we don't have to use burp for this anymore. Some sites check referrers, and user agents, or expect a field from a specific server rendered page to be added to a header. More than one expected a javascript style timestamp on every request.

The weirdest behavior comes from older apps that started as purely server rendered, and slowly added a dynamic frontend. I always cringe when it's obvious that different developers were given tasks over the years, and completed them without bothering to learn the rest of the system.