very quick summary on charge current (with regards to Li-Ion batteries):
a) UM2, the earlier model has selectable charge current from 0.1A to 1.5A (in 0.1A steps), assuming USB-QC power supply
b) UMS2, the later model has selectable charge current from 0.3A to 3.0A (in 0.1A steps), assuming USB-QC power supply
UMS2 also has battery internal resistance reading, while the UM2 does not.
Regarding the UM2 ‘cooking’ NiMh, I doubt the UM2 is really doing that, as it seems the battery used are old (too high resistance) and probably problematic for many chargers as well.