I have had all sorts of grief with the * shorthand. I always write out the full code using <template>
elements, and that might help you here.
<template ngFor let-post [ngForOf]="posts">
<ion-list>
<template ngFor let-comments [ngForOf]="post">
<template ngFor let-comment [ngForOf]="comments">
<ion-item></ion-item>
</template>
</template>
</ion-list>
</template>