The rules to either multiplayer label, or not, based on multiplayer testing:

Y = yes
M = no, but might work if board state was resync'd to update values of affected pieces
N = no, host only

AbilityActionCostAdjustedRule.cs - Y, but info isn't updated on card
ActionPointsAdjustedRule.cs - Y
CardEnergyFromAttackMultipliedRule.cs - Y
CardEnergyFromRecyclingMultipliedRule.cs - Y
EnemyDoorOpeningDisabledRule.cs - Y
EnemyRespawnDisabledRule.cs - Y
GoldPickedUpScaledRule.cs - Y
LevelPropertiesModifiedRule.cs - Y
StartCardsModifiedRule.cs - Y

Note: The following chunk can now be considered safe with the introduction of #151.

AbilityAOEAdjustedRule.cs - M
AbilityDamageAdjustedRule.cs - M
EnemyAttackScaledRule.cs - M
EnemyHealthScaledRule.cs -M
PieceConfigAdjustedRule.cs - M
StartHealthAdjustedRule.cs - M

Note: The following are still not considered safe for multiplayer environments. For sure, CardLimitModifiedRule seems impossible given current client-side code. The other two could probably be reworked using another approach, but doesn't seem pressing.

CardLimitModifiedRule.cs - N
CardSellValueMultipliedRule.cs - N
RatNestsSpawnGoldRule.cs - N

Closing this as it seems to have been resolved. Feel free to open another issue if piece stats aren't syncing up as expected, or something non-multiplayer safe needs addressing.

CC @jimconner @Buruko