Most "resin" composite fillings are made of polycarbonate (methacrylate) that contains a chemical called bishenol-a, a xenoestrogen. Look up bisphenol-a. They also contain various other stuff like formaldehyde and aluminum oxides and barium etc. They are far from being "simple inert plastic".
As I understand it Diamondcrown brand uses polyethylene, a plastic that is safer and does not contain bishenol-a. Good luck finding a local dentist who uses it though.
I'm not aware of any filling that is 100% safe. Our society values marketing and special interest dollars over health and common sense science.