The 3 stars are rather generous, you need to be lucky to get one that isn't problematic. The main issue that all these low-cost clamps generally have is that they don't clamp completely vertically because the part that does the tightening and moves up and down doesn't fit well. As a result, when you tighten, the parts you need to glue move. So you have to use auxiliary ones, after you have used the 'good' clamps.