While I'd agree that uncontrolled DM can potentially be a high risk diagnosis, I don't think it necessarily always is one, so I would not assign a 'high' level of risk just based on the diagnosis. I think there needs to be some kind of documentation by the provider in the record to support this, such as by indicating in their assessment that the disease was severely exacerbated or progressing to a level that involved a threat, or reflecting in their orders or changes to the treatment plan that the patient is in fact at high risk. I'd point out that the CMS guidelines say that "assessment of risk of the presenting problem(s) is based on the risk related to the disease process anticipated between the present encounter and the next one. The assessment of risk of selecting diagnostic procedures and management options is based on the risk during and immediately following any procedures or treatment." This suggests to me that high risk needs to be a more urgent and immediate factor than simply what the disease may eventually lead to in the long term. So if the provider is just making some changes to the care plan and having the patient follow up again in 3 months, for example, this would make it difficult to argue for an assessment of high risk just based on a diagnosis of uncontrolled DM alone.
Incidentally, I might mention that when I coded for an endocrinology clinic, in discussions with the providers I found that a lot of their visits with diabetes patients were both time-intensive and involving a lot of patient education, and in these cases we found that time-based coding worked better for these complex visits than trying to support a higher level based on the history, exam and MDM.