I strongly suspect the mixed-ability class arrangements works when the band of abilities is not "too" wide, and especially when the lower ability segments are not linked to behavior-based issues leading to behavioral classroom disruptions. Too often, I see little to no emphasis within these arrangements to link intensive mental health and other support infrastructure counseling to assist students and their families, and manage behavioral issues before introducing those students into mixed-ability classes.
A big advantage I see in mixed-ability classes where behavioral issues are managed ahead of time is the opportunity for the students who grasp a particular topic sooner start teaching and explaining it to their peers who don't grasp it yet, which cements their understanding, and this can continue onwards down the age and and ability spectrums. There is nearly nothing quite like having to teach a topic to really reveal how well you grasp it.
A big advantage I see in mixed-ability classes where behavioral issues are managed ahead of time is the opportunity for the students who grasp a particular topic sooner start teaching and explaining it to their peers who don't grasp it yet, which cements their understanding, and this can continue onwards down the age and and ability spectrums. There is nearly nothing quite like having to teach a topic to really reveal how well you grasp it.