It's more because the UVB can be very strong and the heat too high. The UVB can be dangerously high. On the flip side, because of the heat, the bulb and chameleon may be so far away from each other that the chameleon cannot get any UVB without burning itself. Plus, like you said, with an all in one type bulb, the chameleon can't just bask under the UVB without getting heat like it would be able to to with a separate UVB and basking light. Cages also tend to be too small to allow enough space between the animal and the bulb without keeping the bulb really high above the cage (which can be a big pain in the butt.)
They are fine to use if you can measure the UVB with a meter to make sure it's working out all right. Without a meter, you really have no idea if the UVB is adequate.