I think Matomo is quite similar to Google Analytics which many people feel is bloated and confusing from the user's perspective. The idea with Plausible is to simplify web analytics and make it more understandable compared to what GA/Matomo offer.
Granted, Matomo does have more depth and features in some areas. It can be the better choice if you want to go very deep into analytics and need some power features that Plausible might not support.
We wrote a little (clearly biased) comparison with Matomo[1]. I hope we're not too harsh on it because Matomo is a great project and still a good fit for many people. But obviously we feel like a modern and simplified take on web analytics fits better for the majority of website owners.
PostHog is more focused on product analytics rather than web analytics. It's a very different product to Plausible so we didn't really do a comparison with them. I would say they're more of an alternative to Mixpanel, Amplitude and those type of products rather than an alternative to Google Analytics and other web analytics tools.
I have used both and stuck at Plausible. A few reasons (subjective):
1. Plausible is GDPR compliant by default, it has an effective way to measure analytics throughout the day without cookies
2. It is simple and that's key. I don't need to know much, Plausible just gives me that
3. It's fairly lightweight. Matomo is quite heavy and as my VPS'es are pretty much scaled down, less is just more
4. The Plausible self-hosting doc is centered around Docker, which is the architecture I use myself and is set up in literally a few minutes