My guess is the effects will largely be related to endocrine disruption and increased oxidative stress (which we have seen in fish exposed to microplastics). Plastic tends to “adsorb” pollutants, which are then released into the body on ingestion, and are full of toxic additives. While plastic is fairly nonreactive, microplastics have a huge surface area:volume ratio, which means a much, much higher proportion of the plastic is in contact with our tissues/system/whatever.

So yeah, hormonal stuff plus damage to kidneys/liver would be my first guess, no doubt plus loads of other lovely things