It's interesting because I can't think of a single death metal or black metal band that wouldn't be considered extreme, but the label doom seems to cover such a diverse range of styles from sludge, funeral doom, drone, death-doom etc. which I'd call extreme to stoner rock and traditional doom which are closer to hard rock and not that extreme.
A lot of the academic writing on extreme metal completely ignores doom or only gives it a passing mention, and I wonder if that's because it is so diverse and harder to make generalisations about than other forms of extreme metal.