The only major difference between Hold and Mountain Mode is that MM will push power back to the battery if it is below the set threshold. Hold will only maintain the battery at the SOC when it was engaged.
Ideally, you want to engage MM before you really need it, because it will make the engine work harder to build up the charge to the set threshold.
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"The balance of power is the scale of peace. The same balance would be preserved were all the world destitute of arms, for all would be alike; but since some will not, others dare not lay them aside. Horrid mischief would ensue were one half the world deprived of the use of them; the weak would become a prey to the strong." -Thomas Paine