Thats very fair. Both had one great season where their style became accepted because of the level of their performances. Id chuck Valencia in when he was first a winger here as another limited player who served a tactical purpose, again got a hard time of it. You need these players as they do a job.
Lingard will probably end up being the same. Favoured by the manager for his work rate but receives abuse when he's not having an amazing game. The contributions to the team are hardly ever recognised.
Valencia was a beast during his best years on the wing though - especially when it came to productivity. Welbeck/Park could, overall, get away with their lack of end product because other players (Rooney, Ronaldo, Tevez, Berbatov, RvP etc.) were producing at that point.
But when your team and attack is misfiring, it's nigh on impossible to have someone like Lingard in the team and not expect him to produce, if that makes sense. Especially when there are players like Martial/Mkhitaryan who are not being utilised, or given an opportunity to play themselves into form. But that's also a case of Lingard now getting abuse for JM's managerial decisions. Which is also unfair on the player. Don't get me wrong, I personally don't think Lingard or Fellaini is good enough to start for us - but they do have their uses.
I think the final point which is very important to realise these days, is that the league is so competitive at the moment, and around the rest of the Europe, the top teams are now littered with talent in every position. So it's also become a little more difficult to accommodate players in your team, especially attacking ones, who are there mainly because of their hard work. That's the difference between ending top 1-4, or ending out of the top 4. All just IMO, anyway.
Sorry. I'm derailing the thread now I think.