Of course deep passes are more hit and miss than short passes. Just like longer FGs are less accurate. A similar QB comes to mind in Ben Rothlesberger. He throws deep and the percentage of completions is far less compared to when he throws shorter passes.

So Yes, not every deep pass Goff throws in on the money. That's the nature of deep passes and why some teams shy away from them.
For near perfect accuracy do what KC does; never throw longer than about 25 feet (yes I'm exaggerating). Then you'll have a QB with about a 70% completion rate. That Goff is the in 62 -63% accuracy rate considering how many long balls and long-intermediate balls he throws is quite good.

What a deep pass does regardless of whether it hits or misses is that it causes the D to have to defend deep and thus open up holes in the intermediate routes (which is exactly where Woods lives and thrives).

Goff is a pretty good deep passer. But more important, he is willing to throw a deep ball knowing ahead of time that the odds of completing the pass drop significantly from checking it down. It carries a risk. That is also a credit to McVay who obviously encourages it.

So, yeah, he'll have his games when the deep ball just misses, and some where they hit. Just like the old Mad Bomber himself, Darryl Lamonica. I suspect Goff's long ball accuracy will improve a bit over time.