The reality is that we do not know what the long term effects are. But nonetheless, without knowing what the consequences are, we managed to contaminate most of the planet including our food and water supply with them. Thats what is worrying because there are many cases where we did something similar with disastrous consequences.

A recent study indicated they appear to contribute to strokes and heart attacks.

https://www.theguardian.com/environment/2024/mar/06/microscopic-plastics-could-raise-risk-of-stroke-and-heart-attack-study-says

Exactly my take. "Plastic" is a huge word covering thousands of compounds. I'm no chemist, but all the plastics I deal with day to day are inert, which is much of the reason we use them.

Plastic at the microscopic level, if they aren't doing anything chemically interesting, really ought to function about like "rock, but light". Most organisms don't run into trouble because there are tiny bits of rock in the world, so I would expect tiny bits of plastic not to be a huge problem. Which is sort of backed up by how we have noticed microplastics everywhere and we haven't seen huge problems resulting from it (most people are still alive, most children still develop to adulthood, etc.).

But it's entirely possible that some of these plastics are not chemically inert, and that they emit chemicals that do exciting and unwanted things in people's bodies. If we can't keep our plastics from becoming microplastics, we probably need to discontinue the manufacture of non-implantable plastics, since all the plastics will end up in someone's body at some point.

And it's also possible that the microplastics physically do do something interestingly bad. I think there was a recent study to this effect on heart disease. But at this point, that's the question we need to be asking. How many or what kind of microplastics does it take to give a ferret epilepsy? Not "are there microplastics in my all brands of peanut butter?"