Something changes one day, and then your app is broken until you update it, causing you to need to rush to fix it, especially if your app has paying customers. This could be a
massive, sudden change of any kind since you are effectively an adversary. Bad.
You are behind a firewall designed to cut out bots, and now need to bypass or handle anti-bot CAPTCHA. Again, bad.
You want to provide push notifications, but that requires background polling on a server. Yep, that's pretty bad, since it multiplies the effort AND erases the "basically just a browser" part in one swoop. Now you need to probably pass user credentials to your backend (bad) and scrape on the server, bypassing Cloudflare/etc in a headless manner (bad.)
Reddit shuts down old.reddit.com and cuts off access to NSFW and "edgy" or perhaps just small subreddits except when logged in on official apps. Now you need to scrape from either official iOS/Android APIs where the keys could go invalid at any time, or the JS desktop site which is all JS and XHRs everywhere. The only way to make your app truly behave like the real thing is to carefully set up an actual browser engine and hide evidence that you're using automation, which is a lot harder than you might imagine. Yep, you guessed it. Very, very bad.
For that last thing, Twitch's "integrity check" mechanism is a great illustration of just how user hostile things will quickly become. Twitch integrity check won't even pass for real browsers half the time. It hates Linux users. This is the future you get.