If you're in Hold mode with at least a third of the battery remaining, there's no reason to switch to Mountain Mode.Found another part of the manual with more info on the modes.
Tried Mountain Mode today on a long steep grade. I had been on hold mode saving my battery and I went to MM and it just went up the hill on the battery, never started the ICE.
It doesn't have any problem on battery, I like to use hold but it feels under powered sometimes.
All Mountain Mode does is rebuild a third of the charge and hold that level with the ICE for those places where you need a sustained output of more than the engine can provide.