BPS is a way of seeing health/illness/disability. It is a general model.
So as a comparison you have the Social Model of Disability and the Medical Model of Disability. People who agree with the Social Model see all Disability through that lens, which is about a lot of the difficulties disabled people face being located in Society (lack of adaptations, prejudice, but tending to neglect the bodily impairment itself). In contrast, Medical model people see Disability as located in the faulty bodies of individuals, it can be quite a paternalistic approach (the approach which led to lots of disabled people forced to live in large institutions).
People don’t tend to apply different models to different conditions, it’s a way of seeing the medical/disability world.
BPS is a lens like this, so it is predictable that BPS proponents will tend to think that it applies to everything (cancer and diabetes as well as ME). What probably varies is the emphasis on the different aspects (bio vs psych vs social) and the plausibility of pushing the psych side. In ME it’s clearly applied as bioPSYCHOsocial but in cancer it’s probably BIOpsychosocial. I’m not up to date on how they’re applying it to cancer, but social aspects are plausible- eg the laws/acceptability around smoking, asbestos and tanning beds.
There’s definitely an element of patient blaming with diabetes and decision making in terms of diet choices means a psychosocial side can be brought out and importantly the public will buy it. Patient blaming is the major flaw of the BPS lens (especially if there are political and financial biases). The complexity the BPS approach should have brought in could have been an advance on how we understand health. Unfortunately it is cheaper and easier to get the individual to do CBT and make different diet choices than to invest in biomedical advances or address complex society level issues
[Also, while on this topic, none of the current models of health/illness/disability seem to be a good fit for our experience as people with ME or other Spoonie conditions. Which also means the professionals we interact with likely don’t see us through an appropriate lens]