Japanese patent publications are typically considered to be fairly safe prior art references, as long as you prove up authenticity and offer sufficient evidence of publication.
But it turns out that that second part—showing publication—is kind of important.
In F'real Foods LLC v. Hamilton Beach Brands, Inc., C.A. No. 16-41-CFC, Judge Connolly excluded a Japanese Patent Office Utility Model Publication on a motion in limine because the defendants failed to show that it was publicly accessible under § 102, based largely on defendants' own position in opposing IPR estoppel.