The ball bearing spindle larger machines I have seen in my area come from companies like Electric Boat, and the nuclear/military facilities.
The ball bearing spindles create rounder parts with better surface finish then Timken bearing spindles.
The lathe here with the flat belt drive could produce outstanding surface finish, if working correctly.
The Pratt&Whitney model C was available with high speeds and ball bearing spindles, a geared head machine designed for high speeds of the day, was used in the gov, or big defense contractors locally.
The best tip I ever got on ball bearing spindles is, if you have take them apart "clock", or mark all the bearings inner and outer races, so the bearing high marks, or replacement bearings high marks can be returned to the spots where they were, on the spindle shaft, and housing, its only logical, and cant be argued with.