[polote]

There are also users who bounce before js has the time to execute. Which could easily be 10-20% of users. (GA is usually loaded via GTAG manager, so not executed at page load)

Even among js analytics tools, there are differences. In a past job, the main question customers where asking, was "why there is not the same number of pageviews than GA?".

Everyone detect bots differently, blacklist different IP, js execution orders is different and some script are blocked by adblockers and some not