Drs Frankel and McKay first note that the FDA is able to ensure biosimilar and reference biologic quality through prerelease specifications. But these specifications cover only a subset of all measurable quality attributes. If FDA testing were adequate to predict all emergent quality-related safety issues with biologics, then there never would have been cases of pure red blood cell aplasia associated with Eprex1 or HX575, which are biosimilar versions of epoetin-a, nor would have there have been deaths due to contaminated heparin in the United States in 2008.
Chandra, Amitabh, and Jacqueline Vanderpuye-Orgle. "Competition and Biosimilar Products--In Reply." JAMA: Journal of the American Medical Association 314.24 (December 2015): 2692-2692.