It's compatible with Old School Renaissance and original D&D monsters, dungeons, and adventure modules.
Let's take a hill giant from 1e:
AC 5, HD 8 (i.e. also attack bonus)
How does the skill system from WoD work with those values? Do I take AC as armor points and light weapons (d6) only do damage on a 6?
And the other way around: does the hill giant always hit me (lowest roll 2 plus 8 attack bonus >= 10, always a full success)?