Some meal plans are bad because they are meant to be temporary. They’re designed to help a person get to a specific short-term goal, like dropping a few extra pounds before a wedding, learning to manage blood sugar, or cutting weight for an athletic competition. Our bodies can usually adapt to a rigid way of eating for a short period of time. But if you’re too strict for too long, you could wind up with disordered eating habits and lasting health (mental, metabolic, hormonal, etc) consequences.
Eventually, you get so turned off by the process that you regress or quit altogether. You conclude that “eating healthy” is boring.