WAR is dung, concur. Particularly fWAR.
But looking at just batting average also misses a great deal about a player's offensive production. At a minimum add On Base Percentage and Slugging percentage.
Wallner comes out better when you consider those aspects, although he spent a lot of this season really struggling.
The strikeouts are a problem, contributing to his slumps and really limiting his RBI.
Still and all, he's nowhere near the biggest offensive hole. And IMO he's been taking much better ABs for a while now.
Of course, slumps and streaks are probably always going to be part of his game. Too much swing and miss does that.