Since the minimum rank to review the mixer service still really a low rank "Full Member", and @OP can still get a spam attack for a review reward because of the minimum rank.
I suggest for the @OP to check each participant at other threads about "offering a member to review mixer service", you can use/see this thread as a reference
https://bitcointalk.org/index.php?topic=5140688.0 to selected participant review. A lot of people who participate at there were really given a good review and they have knowledge about the mixer service.
So you just not wasting the fund, selected a wrong member who just gives a short review like "Good mixer, the deposit & receive coins was really fast, etc". Since i don't have a knowledge about the "Mixer Coin" I'm not to participate at the review.