What does he offer then from the purely attacking point of view? His passing is safest you'll see on the pitch, his dribbling is one dimensional and even when he dribbles past player it mostly ends up with him crossing the ball(and we know how that ends). He is better defender these days than attacker, and he is used almost as a winger in our set-up.
A fullback's most important role in an attacking sense is building and sustaining pressure, by bringing the ball high up the pitch and making sure it stays there through either recycling possession or pressing high. Pretty boring stuff to analyse and it doesn't do much to override the frustration from wasted crosses, but that's the reality. Valencia's very good at that.
Many of the best fullbacks in the history of the game weren't very good crossers for a reason. Many of the best attackers in the world, decade after decade, weren't very good crossers either. It's both a very difficult skill and not a particularly productive one. The boring stuff is much more useful.
He lacks the technique and imagination of someone like Carvajal, never mind someone like Marcelo, so it's undoubtedly true that he's not at a top tier attacker. I just think people vastly overstate the number of players who are or were at that level, alongside vastly underrating the actual essentials of the role.