Is there any significant empirical evidence showing drawbacks of ACT therapy? According to The Association for Contextual Behavioral Science, Acceptance and Commitment Therapy (ACT) has a vast database of empirical evidence showing it's benefits. Based on this evidence, it seems that this type of therapy should be the primary option for people who struggle with any problems.

This is seemingly illogical to me. After all, if this therapy has no drawbacks, as it seems, I don't see why it wouldn't it be used for everything. Are there any empirically based claims showing the drawbacks of ACT therapy?
After significantly more research, with help from the comments I have found the following:

According to The University of York,


  "Acceptance and commitment therapy was more effective than control conditions across several disorders, but there was no evidence that it was more effective than established treatments." 


More specifically, they state: 


  "[A]cceptance and commitment therapy was of statistically significant benefit for depression (two RCTs), physical health (five RCTs), and other mental health conditions (seven RCTs), but not for distress conditions (four RCTs)." and "When all trials were combined (regardless of comparison or duration of follow-up), acceptance and commitment therapy was significantly more effective than control conditions"


This evidence points to ACT being equally as effective as established therapies, but ineffective on distress conditions. As Fizz pointed out in the comments, the biggest issue with ACT and other third-wave CBTs is that they are over hyped as miracle solutions despite little empirical evidence showing they are more effective than already established methodology.