-50% Land Losses on Ambush Attacks
The WH bonus affects acres, not combat losses.
It's a useful bonus, but by itself is not enough to make WH competitive. Assuming the WH isn't chained or fireballed to neutralize most of it's bonuses - and WH would have to be run as a kingdom-wide strategy - is faster training and drafting going to be much stronger than OME or attack time? Probably not. WH doesn't get the means to accrue honor without non-war heroes to help, and because of that the WH in a kingdom are giant bulls-eyes to be chained sooner or later.