I think I can answer this for you
1. If a product has less than 3 reviews on it, it can be requested. However, sometimes, the 3rd assignment may be out and not reflected on the product page. We just have to roll with this.
2. It's in stock. We can't assign an out of stock product.
3. It already has a description review on it. If the product doesn't have its initial review on it, it can't be requested as an assignment.
4. Lingerie and Costumes are NOT available for review requests.
Hope that helps!