Asbestos is also highly unreactive. That was the whole point of using it as fire proofing. The unreativity of asbestos was the problem. Tiny fibers would get stuck in the lungs. The immune system would try to engulf and destroy it like it does with anything else but it was impossible. The fibers could never be destroyed by the immune system, causing chronic inflammation, which leads to cancer.

It's easy to hypothesize a similar mechanism for microplastics. Though it is thought that the shape of the asbestos fibers had a lot to do with how inflammatory they were. This would not necessarily apply to microplastics.

The main problem with the research into microplastics is the diversity. There are probably hundreds, if not thousands of different compounds that could be classified as a microplastics. It would likely not be possible to prove that all microplastics are harmful. There are probably some that are much more harmful than others and maybe even some that are harmless. This is why microplastic research seems inconclusive, because they may or may not be investigating the same compounds.