Rashford was criticised for missing chances, despite getting into good positions.
Martial's shortcomings are different - he doesn't seem to have either the instincts or desire to get consistently on the end of chances in the first place. It takes two to tango, and if he's not there to be a potential receiver in the box, it makes our creators look even more impotent than normal.
Which isn't to say a centreforward's only job is to score goals. We got to a CL final with Welbeck as our starting striker. Sometimes all you need is a focal point who occupies defenders and drags the opposition backline out of shape for his teammates to take advantage. Rashford was at least doing that much earlier in the season, whereas Martial is a step down from even there. If Martial is not working for himself, he should at least be working for his teammates - the fact he isn't is where the frustration comes in.
Martial only seems to activate when the ball is near him. The biggest issue is his off the ball movement. And when I say off the ball movement, I don't mean his pressing. Those are two different things, but seem to get conflated under the idea of being hard-working.
If it's his innate fitness levels that limit him from improving, then he's not cut out for elite level football and fair play to the lad for living up to his potential as much as he has. But if it's his attitude, he needs to change it because he deserves to be called out on weaknesses that are his own fault.