Microplastics (MPs) have evolved from emerging pollutants to one of the most intensively studied environmental stressors, yet this rapid growth has not been fueled by methodological consistency or environmental realism. Many studies continue to apply arbitrary polymer selection and short-term oxidation protocols to simplify experiments, which produce results that are difficult to compare to natural systems. As a result, the environmental fate and toxicity of MPs remain poorly understood, and their role as vectors for chemical pollutants is still debated. This review critically examines these limitations and develops a framework that links polymer chemistry, additive composition, and environmental aging with toxicity and bioavailability. It demonstrates that factors such as additive leaching, realistic aging conditions, or multi-contaminant exposure are often neglected, which leads to distorted interpretations of MP-pollutant interactions. By compiling findings across studies on sorption, leaching, inherent toxicity, and co-contaminant toxicity, the main sources of inconsistency and bias that hinder comparability between studies are identified. The review emphasizes that MP behavior cannot be captured through single-factor, short-term experiments but instead requires coordinated, standardized approaches that mirror natural heterogeneity. The proposed framework advances the field toward more reproducible and environmentally grounded research capable of supporting credible risk assessment and evidence-based environmental policy.