I think "strict" becomes unhealthy when people start getting wound up in knots over every detail of their food and begin to really get stressed by it. I'm strict by many people's standards because I'm celiac, so the indulgences in beer or other foods that contain gluten are off the table for me, but I'm not stressed out over it because it's part of me being healthy. The harm of eating gluten for me is quite serious, and being strict about it means less stress for me rather than more.
I do worry when I see people who are so freaked out over nutrition that they end up in a kind of food paralysis and go into crisis mode over any possible primal impurity in their diet. It's not really about the strictness of a person's diet--it's about the damage they're doing in getting so stressed out over the minutiae. You see those people who are in a tizzy because there might have been a touch of canola oil in the salad dressing, and I have to think that the stress is doing more harm than the canola (except maybe for someone who has an allergy issue, of course).
So no, the strict diet isn't the problem; the panic mode is.