There is a basic ~9:1 gear reduction between the rotor (the "motor" part) and the differential gear. Other than edge cases (track, abuse) the LDU gears are stout and rarely give issues. The weakest gear are the differential side gears, which some have managed to break, enough to note, but overall the "gearbox" (three gears) don't give any trouble.
The rotor bearings, OTOH . . .
a) The original rotor bearings used conventional steel alloy balls, which became an electrical path from the rotor to the chassis, and became prematurely worn. They became noisy, and eventually allow enough shaft play to trigger a coolant seal failure. I think this affects both rotor bearings. A mechanism that Tesla employed to avoid this problem, an "Aegis Ring" brush setup, did not work as effectively/as robustly as they'd planned. Later Reman LDUs used rotor bearings incorporating non-conductive ceramic balls, and removed the Aegis Ring.
b) The rotor's coolant seal fails. Often. It can allow coolant to dilute the outside rotor bearing's grease, causing failure of that bearing specifically. Many different revisions of this seal/rotor interface have been documented over the years, but the bottom line is that coolant in the rotor cavity eventually ruins the LDU, rendering it un-repairable.
So, if a check of the speed sensor yields a dry sensor, you likely have a repairable LDU.
If it's wet with blue coolant drops, you are running on borrowed time and risk the LDU becoming scrap/for parts only. The rotor can acquire enough rust to prevent it from turning at all, and the coolant can and does migrate to the "other side" of the LDU and fry the power electronics.
A 4-minute video by QC Charge on checking the speed sensor for the presence/absence of coolant
A 12-second video by alflash showing a wet speed sensor, with assembly grease and/or rotor bearing grease debris attached.
The sensor should not have those wet dots (the tan grease may or may not indicate an issue by itself), and if you blot the wet sensor with a paper towel, if there's blue, you have a coolant leak. Trouble.
