Helicon focus rail vs manual rack, the macro difference shocked me

Been doing lens and sensor cleaning for years, but macro gear repair always tripped me up. Last month I had this old 90mm Tamron on the bench with a sticky focus helicoid, and I kept getting soft test shots after reassembly. I borrowed a cheap manual rack from a buddy in Austin, then tried a used Helicon rail I picked up for $140. The stepper motor moved in such tiny steps that my focus stack test came out crisp edge to edge, no more fiddling with my shaky hands. Manual was fine on a tripod at 1:1, but at 2.5:1 the manual rack just slipped and jumped. Anyone else notice that motorized rails reveal small alignment issues you'd never see otherwise, or am I just hitting bad luck with older gear?