The froghemoth (VGtM, p. 145) has the Shock Susceptibility trait:
Shock Susceptibility. If the froghemoth takes lightning damage, it suffers several effects until the end of its next turn: its speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, it can’t use reactions or Multiattack, and on its turn, it can use either an action or a bonus action, not both.
However, it also has lightning resistance. That means using lightning attacks/spells will result in reduced damage for the attack/spell, but it will have other negative effects on the froghemoth.
From the perspective of a monster builder rather than a player: How useful is using lightning damage against the froghemoth, and how much will Shock Susceptibility reduce the combat performance of the froghemoth?
To make sure this is not too broad, let’s say we have four 10th-level characters: one wizard dealing the lightning damage (or the same amount of another damage type that the froghemoth is not resistant to), and 3 other characters with about 16 AC and making 2 weapon attacks per turn each.
What I’m trying to find out, is, if, knowing the effects of Shock Susceptibility, there would ever be a reason to forfeit dealing lightning damage over another damage type. I suspect that there generally wouldn’t be and that the removal of the froghemoth’s Multiattack is the dealbreaker, but I’m unsure on the impact of the other effects.
Related question asking about lore and intention of the feature combination: The froghemoth has both lightning resistance and Shock Susceptibility; is this an error? Are there other monsters like this?