Changing the UA only changes the behavior of the web server, so unless that's some block of untouchable code owned by some other team made to 'optimize' page loads for different clients, you're not doing yourself any favors by changing it in your browser; you should be loading up the client itself since you can't simulate the actual engine loading your web app differently.
> If you'd rely on feature detection, your critical path for "showing the initial screen of the app" is bound to "download feature detect, parse it, execute it, fetch correct variants of your js/css packages, parse them, execute them, show screen". That's not cool, imho.
Even for barebones web apps, your initial html response should have either empty data (but otherwise a visible UI) or a SSR'd version of the page shown while the JS downloads and executes.