[philipwalton]

There are tons of things GTM can't do currently, unless you're willing to write custom code. And if you're writing custom code, I'd personally prefer that code to be in my app, under version control, and running through my test suite rather than in a GUI that can be modified by anyone with GTM access.

Here are a few examples off the top of my head of things you can't do with GTM today (without writing custom code):

- Tracking when (and how long) the page was in the visible vs hidden state.

- Tracking anything performance-related.

- Tracking when DOM elements are visible in the viewport (via IntersectionObserver).

- Tracking the active media query / breakpoint.

- Tracking social widget button usage.

- Tracking the use of service worker

- Tracking interactions with native web push notifications.

I think GTM is great for marketing websites, but for better understanding how someone is using a complex web app, I think it makes more sense to have that logic in the application code.