Before we start this discussion let's clarify what IIFYM and clean eating actually is.
Clean eating, although open to interpretation, is essentially a diet consisting of non processed 'clean' foods. Usually utilised by bodybuilders, the average clean eating diet consists of lean meats (chicken, turkey), fish, nuts, veg (especially green veg such as broccoli, spinach, etc), fruit and complex carbohydrates such as oatmeal and brown rice. However as earlier mentioned, it is open to interpretation and you will get some who only see raw food as clean or there are also those who claim clean eating is a myth.
IIFYM (If It Fits Your Macros) on the other hand is an approach to dieting where you work out your calories and macronutrients (protein, carbohydrates and fat) based on your goals, activity level and body composition and you make sure that each day what you eat fits within those calories and macros. This allows some flexibility in your diet - if you eat a piece of cake at lunch you will then reduce the carbs and fats at dinner making the cake fit in your macros.
Does clean eating work for fat loss? Yes of course it does, as long as you are eating in deficit you are going to lose weight. However, that is precisely the point - you could be eating chocolate and fries all day long and as long as you are in deficit you will still lose weight. The problem with clean eating is that it allows no flexibility, more often than not you will have a very limited lists of food that you are allowed to eat and that unfortunately for a lot of people leads to things such as binging (when they eventually fall off the wagon) and eating disorders. This of course doesn't happen to everyone, but it is a common occurrence - due to the restrictive nature of 'clean eating' diets and meal plans, the dieters often develop unhealthy relationships with food.
A big misconception about IIFYM is that those who follow it survive on pop tarts and pizza alone - this is simply not true, most people are sensible enough to fill 75-80% of their calories with whole nutritious food and using junk food and treats as the remaining 20-25%. In my opinion IIFYM really IS the perfect dieting approach - it offers you flexibility meaning you are a lot less likely to binge and fall off the wagon and it teaches you how to have a healthy relationship with food as you will no longer by labelling foods as 'clean' and 'dirty'. And who wouldn't want to be able to indulge a little in their favourite treats while losing fat?