[T 2] Stacking of transgenes is yet more difficult than stacking natural genes, but especially in the case of pest resistance genes which require a significant financial investment to insert, is advantageous over other methods.
[1] Pathosystems with rapid evolution in the pathogen have long been considered good targets of stacking, to broaden and prolong resistance.
[2][3] Assaying for successful insertion of R genes is much more difficult than for one at a time.
[4] A simple challenge assay will only tell between complete failure/some unknown degree of success.
[4] Building a new technique specifically for the multiple genes you are attempting to insert may be necessary.