What is the biggest problem with GMOs?

Could you please elaborate on the primary concern surrounding Genetically Modified Organisms (GMOs)? Could you possibly explain what makes it such a significant issue? Is it the potential health risks? Or is it the environmental impact they might have? Furthermore, how do these concerns compare to the potential benefits GMOs could offer? I'm interested in understanding the balance between the two sides of this debate. Could you provide some examples or case studies that support these concerns? Thank you for your time and insight into this complex topic.