Basic explanation is that they are industrial products high in oleic acid / omega-6 fatty acids and therefore highly inflammatory. Inflammation in the body is the root cause of all manner of autoimmune and metabolic issues. Then there are risks of oxidation and rancidity which lead to lots of health issues like cardiovascular problems.
On an environmental level, the monocropping involved in production and the processing of thr oils destroys ecosystems and pollutes water systems.