There are some distinct strengths of using Immunoassay methods for protein detection. They provide a direct measurement of the biologically active protein of interest. They are validatable/reproducible across different testing locations and different proteins. They can also provide a simple presence/absence measurement or in other cases provide more quantitative analysis. Especially with the lateral flow strip method, they are easy to perform and transfer to other laboratories/users. Lastly, and most importantly for importing countries, they are a widely accepted method by regulatory agencies.
However, there are also some disadvantages of using Immunoassay methods that you should be aware of. In the cases of GMO testing, no single method will detect all biotechnology-derived products (DNA or protein-based). Also, methods are event specific and must be validated for importing/exporting markets. When testing for several different proteins at one time, the different antibodies may cross react. Finally, as with any protein detection system the sample must be taken accurately.