Is it a myth that if a male has undescended testes, he is incapable of impregnating a woman?

Yes. Fertility rates in men with a history of treated or untreated testicular maldescent are generally lower than men without such problems. However, with unilateral undescended testis, fertility is very close to normal. If both testes are affected, the fertility rate is considerably lower, but fertility can be increased with assisted reproductive techniques.
Depends! Fertility is virtually normal if there is one normal, fully descended testis. Male is infertile, and incapable of impregnating if both testes remain undescended beyond puberty. Fertility is reduced for originally bilaterally undescended, more so the higher the testes originally ( in groin, canal or intra-abdominal) before surgical correction. Also more reduced the later in age the surgery.