Well first of all, I'm not saying he sucks. He's a good enough MLS striker, nothing more but nothing less.
But he reached 10 goals and 10 assists quickly because he scores a lot against teams that suck in meaningless games. He has one goal in World Cup qualifying, and it's against St. Vincent and the Grenadines. He has five goals in the Gold Cup, one against Cuba, one against Guyana, two against Trinidad and Tobago in a 6-0 win (in a game that I was at), and one against Martinique. He scored one against Ecuador in the Copa America Centenario (which might be the biggest goal he has ever scored for the US). And he's scored four goals in 27 games in friendlies, one against the Netherlands (probably the only really good team he's ever scored against), two against Bolivia and one against Ecuador.
On the other hand, he's played eight games combined against Mexico, Germany, Brazil and Argentina, plus another five against Costa Rica, in other words, the best teams that the US has played with him on the squad (other than that friendly against the Netherlands), and in those 13 games he has a grand total of zero goals and zero assists.
In other words, he's perfectly fine when you are playing most of the teams in Concacaf or in most low level friendlies against countries from around the world. But when you are playing the really good teams he is just simply not good enough. There is a reason why guys like Dike and Hoppe can play and are coveted by teams in Europe and yet Zardes has never been anything more than a good MLS player.