Synergistic mono cards are much better at adding synergy than synergistic duo cards.
So you're saying that a card like hematite golem that synergizes with rage potion is better than some creature that uses to grow? What about an earth creature that gets benefits from being frozen? Or is that forced combo?
Hematite Golem is not very synergistic. Otherwise yes.
If a creature gets benefits from being frozen but has equal efficiency whether frozen or not then it is not a forced combo. (Think of a bonus proportional to the additional cost to freeze it.)
If a creature gets benefits from being frozen but is less efficient frozen then it is not a forced combo.
If a creature gets benefits from being frozen and is more efficient frozen then the unfrozen variation is not an option. Since the only option requires freezing it is a combo. There are 2 ways to freeze your creatures. Since 2<3 it would be a forced combo problem.