Reuben, "unstable as water," would "not excel," because of his sexual depravity. Although he was the firstborn, he did not receive the birthright blessing.
Jacob prophesied Reuben to be "unstable as water," mainly in moral matters, but the wider principle applies to other things. Deborah chastised the tribe for having "great searchings of heart," but never making a decision (Judges 5:15-16). And though a leading and honorable people (Numbers 2:16; 32:17-18), Reuben will not excel because of its character traits (I Chronicles 5:1-2).
