USSEP changes Erik the Slayer's class (CombatBarbarian) to the one that match his default equipment (CombatWarrior1H) but I believe it is a mistake (change made due to
Issue #14739: Bad Skill matching to equipped armor(multiple npc's)).
Cutting Room Floor addresses one of the issue with the hirelings:
All of the hirelings, except Marcurio, had personal outfit entries which were not being used. These have now been assigned. (Bug #21985)
This fix includes Erik, as he is one of the hirelings. In result, CRF changes his default outfit to the light armor, which would match his vanilla Skyrim's class. I see this as a proof, that the vanilla class was the intended one, and as such, class change in USSEP should be reverted and instead a new equipment, consisted of light armor and two handed weapon, should be assigned to Erik as his default.
Also, with above changes, his combat style needs to be reverted to vanilla HumanBoss2H (change made due to
Issue #19324: Erik the Slayer has HumanBoss2H combat style).