Curiously, though, Usener nowhere indicates any deep interest in Epicurean doctrines,
Like TauPhi said last night. this adds a very good "neutral" perspective that allows his translation to be used from the perspective of someone who was focused exclusively on the merits of the text.
Comparing multiple translations is always probably a good idea.
I also have to say that if someone can maintain that they take no personal interest in some of the most important works of Western Civilization, I immediately have questions about either that person's honesty or their judgment. I would rather a person try to be honest about their personal views, whether hostile or sympathetic, rather than rely on claims of objectivity. And it seems we have reason to suspect that Usener had strong religious viewpoints that were not consistent with Epicurus.
And I hope to at some point figure out what his issue with Nietzsche was, as that might also be illuminating. We know that Nietzsche found Epicurus' religious views very important, and it wouldn't be surprising that anyone who has the opposite view, and is friendly toward religion, would take a less positive view of Epicurus.