I think this may be a comorbidity rather than causation type scenario: folks with substance abuse disorders usually have personal histories of trauma/high ACE scores, and those with trauma backgrounds also tend to have more chronic health problems - that kind of stress is very damaging. Discovering undiagnosed conditions at the time of surgery is more likely in low income/uninsured/underinsured patients as well, those who are under stress due to socio-economic factors, and I would think that these conditions being previously unknown and untreated would also lead to worse outcomes.
The anaesthetic and pain pieces are interesting, professionals need to be able to get accurate information about cannabis usage pre-surgery, perhaps by explaining why they need to know and by promising patient confidentiality.