Let’s sacrifice one Lego axle to see how it deforms or breaks when high torque is applied.

The results (using PF M-motor):
1:1 (40 mNm) – ok
3:1 (110 mNm) – ok
5:1 (180 mNm) – slight permanent deformation
9:1 (300 mNm) – serious permanent deformation
15:1 (500 mNm) – even more serious permanent deformation
– (? mNm) – breaks in half