Functional training is all in the results, if an exercise program that you start is working for you, you will notice it as soon as your physical condition starts improving. Some trainers claim that some exercises aren't effective at all, which is not true. Each individual exercise is meant to aim and improve one part of your body at a time. If you have been injured and have had your arm in a cast for 6 weeks then doing bicep curls will help you regain strength, or if you haven't used your core in a long time and have lost strength there then maybe some sit-ups will help you regain some condition. Each exercise should be chosen to focus on areas that you specifically need to focus on. Once you have reached the threshold effect which is where you most likely cannot reach any more improvements unless you just wish to build muscle size. It comes down to personal choice whether you wish to continue focusing on that one part of your body or starting somewhere else that you should improve on.
It is important to remember that your functional strength training that might be just right for you may not be the best functional training for someone else. Everyone is different so what you might need to improve might not be the same for the next person. A training program should be designed to meet everything you need to improve on, as well as the exercises you like and what you want to achieve to make it more interesting and comfortable for you. It's also important to determine which part of your lifestyle are holding you back, if you work at an active job or you sit at a desk all day, what you do in your spare time like shopping or sports, all of these things will tell you what you should continue doing, what you should do more of and what things you should stop doing. Another thing to think about is your age and any possible health conditions that can hinder you from doing certain exercises, each individual is different and has different needs. You should always test which areas you need help in before designing a program for you. This is all important and should be carefully considered when designing a training program specifically made for you, and as your body and physical condition changes so should your program.
Some trainers make the mistake of following what they think will work best, without understanding the exercise movements, techniques and equipment they might be using. A trainer should always base their routines on scientific facts and what is a proven technique rather than adding ones that might be useless and could injure someone. Knowing what is best for someone should be based on knowing the differences and benefits of each style of training, for example whether you should be using high-velocity strength training or slow-velocity strength training depends on what would be best on each individual. It is important to remember that a different exercise or movement may be effective for one individual but not for another, the key is focusing and finding which aspects you should improve first all while being safe and healthy.