A 10% reduction is still nothing to scoff at.
I'll even give you a 15% reduction. Of the non-magical B/P/S damage you take at 20th level. Say it keeps you up for one extra hit. Maybe 1/3 of the time you would have dropped that would keep you up for an extra action, other times you'll still drop before your action. The rest of the time it basically means nothing - either you're down anyway or it's fractional less to heal.
How many combats a day does your Fighter 20 drop? Say a fighter 20 drop 1 in 5 combats. 1/3 of that means 1 in 15 combats it will come in play to give you an extra attack before dropping.
Actually, I realized that math is assuming it protects against 15% of everything, but really it only protects against non-magical B/P/S. So breath weapons, spells, creatures who have attacks counting as magic, poison, etc. So it's a lot less than 1 combat in 15 it actually makes a difference.
For free, I'd absolutely take it. No scoffing from me.
As a opportunity cost displacing another feat or ASI - well, depends on the rest of the build. In the post I recommended staying away from it there were 3 available ASI/feats. I suggestion of for DEX/WIS (under the incorrect assumption he could take resilient WIS instead of DEX), Lucky, Great Weapon Mastery (to go with PAM), and Sentinel. You may have noticed that's over three, so they already can't get everything I suggested.
I don't see that the minor benefit it gives outweighs the opportunity cost of a different feat at 20th. HAM is a great feat at lower levels, but as average damage increases and damage that bypasses it becomes more common it loses effectiveness. 20th level is, by definition, where it will be the least effective. And at that point other feats can and do bypass it in what they give you for your ASI.
