Kakashi's surname was Hatake, not Uchiha, so it was perfectly normal for Kamui to have trouble aiming. At long range, hitting someone's head when he meant to hit their arm was something he could do now and would keep doing in the future. It was a perfectly normal performance—a textbook example of hitting wherever he pointed.
Yuhara and Kakashi had worked themselves to the bone, and they had dealt the Seven-Tails a heavy blow... Well, not that heavy, actually. The Seven-Tails was indeed injured, and after being struck by Kamui, its defenses had even been breached. However, that did nothing to diminish its combat power.
The Seven-Tails could fire one or two Tailed Beast Balls, and it could still fire a third. In fact, because the pain had made it furious, it was very likely to switch into "annihilation mode" next.
In terms of attack power, defense, and overall combat strength, the Seven-Tails held an absolute advantage. Yuhara had essentially lost his combat ability, and Kakashi was in pretty bad shape too. Leaving Kakashi aside, Yuhara did seem to be "revealing his true form" a little. Hadn't he said he could contend with a tailed beast? He had only managed half of that... He had endured it, but he had not contended with it. Instead, he had ended up lying down.
That seemed a little too harsh on Yuhara. If he had not blocked those two Tailed Beast Balls just now, one of them might have drifted into Konoha... After all, tailed beasts were never meant to be fought head-on. Ninjas were intelligent creatures, so they should use sealing techniques to deal with tailed beasts.
It was fine not to use one's brain, but that required the level of the First Hokage and Madara Uchiha. Only those two could slap