Mental health unfortunately does still have stigma attached to it, unlike most physical health conditions where language is often very different
It is true that there is still unfair and unhelpful stigma, but actually I don't think you can compare it to physical conditions (where there is no depression) here.
Where you have a depressed parent, even if they have been good at preempting it getting serious and seeking support, you are still talking about someone who is needing a great deal of "me" time in the sense that they would be less emotionally available for their dc. For an adopted child the impact could well be significant.
For those not not good at preempting, we are talking about the same thing except more so - potentially the parent not wanting to get up, go out, struggling to engage at all, anger. It really would be disingenuous to suggest that this does not happen in some adoptions, and for any child it could cause "significant harm" (to use the word relating to threshold) even if the child is not removed.
I have absolute sympathy with those who suffer depression and agree that everyone will have times when they fall out of their window of tolerance/flip their lid, but I do think that to suggest that clinical depression leading to months off work is normal or average is not right, and it is also not a reasonable assumption that most people suffering will become expert at managing future episodes. In caring professions time off for MH might be more common than in the private sector than in the private sector, for understandable reasons, including burn out but it is not a "norm".
To bring up any child you are looking at having to help them manage their emotions while also managing your own. With an adopted child the psychological and emotional issues are often harder to deal with for many reasons. By stable, I meant people who are emotionally intelligent who are able to deal with setbacks quickly, knocked off their feet less frequently, good boundaries, able to live within their own personal limitations and so on - more likely to be consistently emotionally available and able to cope.
I am sorry if this sounds negative, but I have seen some of the fall out where things have not worked out as hoped. I am also not suggesting "never say never" and I also believe that depression does not equate to not having a great deal to offer in relationships.