I have successfully repaired a dozen or so Borlotti angle drives that use skew gears. Drive-tab breakage comes from installing the big double-thread mounting nut such that the drive tabs only touch by their very tips on the short ZF extension shaft inside the tranny. This then fatigues the tips after 40 years or so, even with the light loads of cable & speedometer.
You're correct- both types of angle drives are not made to be serviced and the assembly grease used when they were built 50 yrs ago has usually turned into a brick. You can soften the 'brick' up by soaking it in penetrating oil or brake fluid for a week or so, but re-greasing the sealed assembly is a real challenge.
The VDOs including on Mangusta & pushbutton ZFs use right angle crown gears, and what usually happens to them is the cast gear teeth wear out or chip after the factory lube goes away. Haven't found a source for the tiny gears yet.