I've noticed in my game that any plus skill items do not affect any entity summoned (basically anything with health, including stuff like lures and traps, etc). Adding points to the skill through leveling works fine but I see no difference when using plus skill items, with the odd exception of raising summon limits at the normal intervals (such as for skeletons and wolves). Other things I've noticed include that mastery shrines still seem to affect these summons as normal, and the synergies of summons still seem to be affected by plus skill items normally (for instance the "maul" ability for wolves will do more damage with +1 to all nature, but the health of the wolves doesnt rise).
Anyway I've tried to find any posts or comments relating to this to see if this was intended by the underlord mod or if its a bug. Since I've seen nothing of the sort I have no idea if what I'm seeing is normal. If someone can confirm that it is a bug, a link to a fix would be greatly appreciated.