Is it "is" or "are" after "that" in the expression ...X of Y that is/are? Context provided in the question

Solution 1:

Were mechanics plural, you could use are to refer unequivocally to mechanics, or is to refer to efficacy.

However,

Mechanics = mechanics noun, plural in form but singular or plural in construction

Merriam Webster

This means that is/are is preceded by efficacy (singular) and by mechanics (singular or plural). The meaning therefore remains ambiguous and unclear.

One way (obviously not the only one) of avoiding the problem may be to make an emphatic repetition:

“... measure the efficacy of gamification mechanics that is particularly robust in some situations, an efficacy that is not so robust in others.”