In my daily practice with patients, we often have them using meal replacements and food supplements for a specific duration following surgery. However, I try to stress that these should only be used for a period of time and not lifelong. I am probably one of the more strict dietitians when it comes to my views on meal replacements/supplements, but most of it comes from experience.
First off, I won't deny that meal replacements can be successful. This is typically because you are decreasing the amount of calories that you would normally consume at a meal. So, if you usually have a 500 calorie lunch and you start substituting with a 250 calories meal replacement, you should obviously have some weight loss success due to the drop in calories. The big issue with this is, what happens when you get tired of the meal replacement? You get bored or you decide you want a real lunch instead of a bar. Well if you go back to a 500 calorie lunch, or one larger than the calories of the meal replacement, you'll likely gain that weight back. With this math, meal replacements can be an effective weight loss aid, but special considerations need to be taken when transitioning off of those replacements.
My other issue with meal replacements/supplements is that your body is getting the same nutrients over and over again (yes, even if there are different flavors). This would be like eating the exact same thing for a meal every single day with no variation. Our bodies thrive on variation and need different nutrients from the diverse fruits, vegetables, protein and grains that we can provide it. This is why I recommend adding variety into meal intake, even if you don't use a supplement or replacement. Switch up your meals so that your body can get different nutrients.
Now, this doesn't mean that meal replacements or food supplements can't play a role in our overall nutrition. These absolutely have their place in addition to a healthy diet and may occasionally be used as a meal replacement. But it does mean that they shouldn't be relied on heavily for use multiple times per day or as a sole source of nutrition (for the majority of individuals).