To be honest, it doesn't seem that huge. It's +1 per 8 hp of damag dealt.
From my point of view, the objection to these sorts of abilities is that their effect on expected damage output can't be assessed without doing probability calculations that require knowing the target's AC, which is an abstract combat mechanic. It doesn't strike me as very true to the experience of playing a great weapon fighter, who is typically either a wild raging barbarian (who is hardly doing complex calculations in the course of attacking) or a skilled polearm or greatsword wielder who intuitively attacks in the most skilled manner possible (who shouldn't be required to take a gamble on self-gimping).
12.7% better than Great Weapon Fighting without and how much more compared to Defensive Fighting and Dueling?
If two-handed weapons were that effective, don't you think all the ancient warriors of quality would have used them in single combat or small skirmish combat? I think they should have tacked something on like an initiative or to hit penalty for heavy weapons to show their lack of maneuverability in skirmish fighting.