Yes, fullbacks hardly assist at all, and that also applies to wide midfielders too.

Wide midfielders could do a bit better when their passing and crossing attributes reach 130% (vs opponents quality around 90%). That’s also why I have both first-choice 130%+ ML/AML(R) having passing & crossing of 180%+ to keep creating those cross&score chances. Also why sub wide midfielders only train with fast counter attacks whenever conditions available for training is 4%+.

Personally, I train players with only central roles or wide flank roles, with the exception of AM having all 3 AM roles (best attacker to exploit opponent’s weakest area), so that’s why I tend to give a down-arrow to fullbacks to minimise them joining attacks and waste crossing chances. Their presence and positioning on the flank do increase attacking frequency on the flank when we put extra bodies to overload.