Privacy, for one.
Google Analytics relies on a cookie controlled by Google, and is likely illegal under the European Union's GDPR. You are supposed to get consent from users before setting a third party cookie, which would require you to throw up a splash screen asking for consent (and just generally annoying visitors to your site).
By contrast, Plausible Analytics is a privacy-first analytics tool that does not use cookies, and respents user's privacy. Plausible is fully compliant with GDPR, CCPA, and PECR.
For two, blocking.
Lots of people block tracking scripts via their browser or plug-in, but many of them don't block Plausible, because it's cookieless and doesn't track individual users.
For three: speed.
Plausible Analytics' system is fast, won't slow down your site, and is an open-source company.
As a bonus, Plausible is also independent, self-funded, bootstrapped and debt-free. In other words, they are independent creators just like Oupost clients.
You can learn more about Plausible on their website. Just as a note, Outpost's group buying power means you get to use Plausible Analytics at a fraction of the cost you would pay on your own.