I once had to implement deep links for a React Native app and, while I’m by no means a great mobile dev there were plenty of thorny bits. For example, I think iOS doesn’t open a deep link in app if you type the url directly into the address bar. It will only open in the app if the user taps a link. Another complexity is making sure a deep link opens in the app for app users and opens the App Store for other users.
iOS also has two different systems for deep linking, custom protocols (eg myapp://) and universal links (a server side manifest that you register with Apple). These each have their pros and cons.
To be sure, those should be solvable problems for a company with even a fraction of Reddit’s resources.
YouTube links, on the other hand, never work for me. They always open in Safari and then I have to find a way to trigger the app link by opening one of the menus on the YouTube website.
The same thing happens to me with Youtube, but only on iPad, not iPhone. The way I get the app open is, when I get to Safari, I try to open the page in desktop. Youtube app kicks open then.