This can be said for BOTH training and nutrition. The ability to adhere to a training program and diet is the top factor in one's success with it. You could have the most amazing training program and diet plan on paper, but if you can't adhere to it for certain reasons such as the diet being too strict or the training not fitting into your schedule, then that really doesn't matter and you will get sub par results. This is why while I preach nailing the big pillars that lead to succes in ones training and diet(For example for diet eating the right calorie range catering to your goals ie in a calorie deficit if you are trying to lose weight, and getting enough protein to optimally gain or maintain muscle mass, and for training doing the right amount of sets, reps, and weight depending on your training age and goals, focusing on constant progression, and selecting the right excercises for your goals) I also preach approaching things in a flexible matter though, catering to the individual so they can adhere to their training and diet and get the best possible results all around. So while one should always focus on the most important principals of training and nutrition for their goals, they should do so in a flexible manner that allows for maximum adherence and consistency, thus leading to not only the best possible reults, but also enjoying the whole process more.