Presume competence?

I’ve been thinking a bit ability the “presume competence” movement.
Presume competence is often something stated in relation to supports for people with disabilities. It involves approaching people with the presumption that the person does understand your speech and has the capacity to develop more complex levels of communication. In many ways I applaud this approach, in particular with children.
But is their risks inherent with this approach?
Sometimes I feel like I sit in between two camps – there are people that I’ve met that I’ve suggested that the person does understand more than appears apparent (in particular some people with autism). On the other hand, I’ve met people for whom I am quite confident they did not understand speech and their body language and facial expression would be their ongoing communication methods.
Some people may criticise me for essentially judging incompetence. They may suggest that I am the greatest impairment in the person’s life, that I am doing a disservice to the person.
But are there times when it is best to presume or judge incompetence or inability to understand and express in a more complex form. Do we risk an injustice to a person by only seeing what we want people to become rather than how they might present currently? Does just talking to a person who may not understand speech compromise the interaction? Do we increase our compensatory interaction strategies when we don’t presume competence? Do we accept different types of engagement than what we might if we presume that a person can understand everything?
Are there populations for whom not presuming competence is okay such as people who have daily uncontrolled seizures as adults, people with advanced dementia, people who appear barely alert? How does this affect the interaction that we engage in?
How long do we presume competence before we make a judgement that a person doesn’t in fact understand? How long do we take the “we just need to find the right key to open the person” or “we just need to ask the right questions or set up the right accessible response mechanisms”? Where is the intersection between presuming competence and “they understand everything I say” in the absence of evidence that a person does understand?
As professionals how do we deal with an objective assessment of comprehension if we maintain a constant “presume competence” stance? How should be able to have a final word?
Additionally, do we we automatically assume that incompetence is a worse state, that people deemed to have a profound ID are lesser people who haven’t reached their potential?
Don’t get me wrong – it is important to try to find out what someone understands and support them to develop. I don’t want to see people who are showing understanding have these signs ignored and get treated as though they don’t understand. I’ve seen enough people who do understand look at people with exasperation when they’ve high pitched them in speech. But I’ve also seen people who do not appear to understand speech have their body language ignored while interaction partners talk at them.
I don’t think there are clear answers, but I do feel that there is a point at which presuming competence might not be always be the best thing (though I’m sure there might be people who disagree).