It is. With Hernandez we are not very flexible. It just doesn't work against team who defend 80% of game. If you give ball to Hernandez in box, he will score, but first, ball needs to find his way to Hernandez. It's hard to make space when you are surrounded with few defenders. He tries to drop deep but he's not really good at it. And that's why we struggle with teams like Blackburn, Stoke... But Hernandez is perfect for teams like Chelsea, L'pool, Arsenal who likes to play possesion game and play highish def line.
I can't question Welbeck's work rate, he is fantastic in that respect. Also like you say, Hernandez is just basically a poacher, but surely with United we need someone the best of both, especially when we just play the one striker up front with Rooney playing in the hole behind him.
It would be great if Welbeck improved some other aspects of his game he could be absolutely great for United, but at the moment he just seems a player with high work rate and no end product.