I would give to him some time before touch any stat, he barely played for FCB and he surpassed recently a two months injury (and he already hasn't made a propper preseason), so I would evaluate him in desember or january, before some time adaptation and get the enough fitness.
I understand he has only scored a couple goals for Barcelona but they all have superb accuracy and placement. Now, from his Udinese days, his shot placement was also decent. I know, he wasn't the most consistent player of all and did have some "bad games". But when he shoots, he goes for the corners and the majority of his goals hit those areas. Okay, even if you say "that was the past", then let's use his recent games for Barcelona. Even his long shots were either very close to goal if they weren't shots on goal or produced fine saves from keepers.
I think a 78 is a bit low. I'm not saying he needs an update now. But in my opinion, it's up for debate. What better to compare Sanchez to his teammates? Sanchez is only 2 points above Iniesta. Iniesta isn't bad but he's still pretty bad at scoring. But Pedro? Pedro's SA is 85. Pedro has decent placement. But his shots are very similar to Sanchez's. At this moment, there is a 7 point gap between the two. Yes, Sanchez just returned from injury and he's only played a handful of games for Barcelona. But isn't his time in Udinese enough to give this guy at least an 80? Yes, he misses a good amount of long shots but even with let's say... 95 SA, you will still most likely miss a lot of long shots.