Rashford has 10 Goals and 5 Assists in 19 Appearances this season. Of course, he got a hat-trick in one game and he has misfired a lot in the Premier League, but one goal in two used to be considered good and I still think it is, especially when you're chalking up Assists as well. Rashford is actually only one Goal and one Assist behind Bruno (having played a game more albeit, despite very similar minutes overall).
Martial is in a bit of a slump, however, with 2 Goals & 2 Assists in 12 Appearances, but he has been injured more and got a Red Card to boot.
They can both be frustrating to watch as they seem to miss a lot of chances, but I honestly think that if we had Salah, or Sterling, we'd be saying the same thing. They miss so many chances, but still managed to be high in the scoring charts. They should score even more than they do, but they typically score enough to get their team places. Main difference is they play for better teams.
Rashford & Martial I think are very similar, with Martial getting 23 Goals / 12 Assists & Rashford 22 Goals / 12 Assists last season. I suppose we should expect them to improve and I'd argue Rashford is well on course to do that. We have played probably 33% of the season, give or take, so this 'form' Rashford is on should lead to bettering both his Goals and Assists from last term. If he can improve his finishing more, he might get 30 Goals. Would we be saying he is a problem then? Martial definitely does need find his finishing boots soon though, as I'd say he is at danger of being dropped for Cavani for a long period.