Plastic poses no infection risk above and beyond fermenting in glass or stainless. There is some concern that scratches in the surface may provide a "hiding place" for contaminating microorganisms. The reality is that any organic residue that remains after fermentation must be removed by a thorough cleaning, or else you cannot sanitize the fermenter properly. Plastic may provide a better foothold for crud, but a soak in warm PBW solution is generally more than enough to remove all of this. As long as your cleaning and sanitizing practices are up to snuff, then there is no cause for concern with fermenting in plastic.