No SEO effects. If anything there could be a slight advantage to using Plausible over GA because the JS tracker is much smaller, though in practice I would not expect any SEO impact.

Plausible is much simpler than UA, all the reporting is on one page and it has much more limited config options. It also can import your UA data. If you're looking for a simplified GA alternative it's a very good choice. (Full disclosure, I recently published an article on their blog about UA vs GA4: https://plausible.io/blog/ua-vs-ga4)