I found a bad bug in their JS which means that on some pages it just silently fails and doesn't log anything, which means your analytics are even more inaccurate than ever (given the browser restrictions). I was totally broke and I wanted to use their paid service for a few months, so I offered them the fix in exchange for a few months free service (maybe $30 credit?). They told me basically "don't worry, we'll find the bug ourselves one day, we don't need your help."
But I've been in one, where a customer offered "patches", despite our software not being open for contributions. Not only were they inconsistent with our standards, they were hard to read and had some subtle security issues on careful review. I'm still suspecting it was an attempt to plant a backdoor.
In any case, even if legit, it was a lot of work on our side to just review and clean it. Far more than if we just did it ourselves.
This is different for OSS, which should have external contributions as main workflow. Ours wasn't prepared for external contributions.
Maybe the same is with Plausible?
It's more then enough for me.
And a few clients whom I enabled it for, told me they very much liked the simplicity. Less data as a feature!