I have not done the Functional Training certification by ACE, but I did take their full day workshop on Functional Training and Assessments a few years ago. It was a wonderful workshop and very informative, educational, and had outstanding materials. It was very useful for everyday training. I know this isn’t the same as the certification, but the workshop probably covered some of the same materials.
Any ACE certifications and workshops that I’ve done have always been very good. Not sure if this will help you, but the knowledge you will gain from the cert. would certainly help you with your clients.