What if your site is a SPA? You would not know, for example, the time spent on site, what pages are visited, where exactly users leave, if there are client-side errors, right?
This is much less likely to be blocked if you self-host it — breaking requests to your server will break the app, whereas blocking common cross-site tracking services is popular because there are few drawbacks for the user.
I guess your options here are, do collect metrics in JS and hope that whatever the reason that 20% of visitors don't show up in Google analytics isn't preventing them from using your site.
1. Server side analytics
2. Client side analytics for information akin to what you're asking that Server side misses
3. Crash analytics for client side errors
As mentioned though, you're only getting partial info from some of those options. It also gives you a chance to decide which of these you _really_ need and hopefully eliminate anything you don't