I agree with heterogeneous. But why do you think that auggests multiple causes. Commonalities across the neurodivergent spectrum intuitively lead me to suspect the opposite, that there is some common mechanism underlying all of them.
Have you read this paper: https://www.nature.com/articles/s41398-019-0631-2
They find that the four-way distinction between ASD, ADHD, OCD and "typically developing" has poor correspondence to the observed interaction between cortical morphology and behavioural symptoms.
If you take their clusters – https://www.nature.com/articles/s41398-019-0631-2/figures/3 – as "what is really going on in the brain", then the same "what is really going on in the brain" can produce all four of ASD, ADHD, OCD and typical development, at different proportions (clusters 1-5). Whereas, conversely, there is a subgroup of ASD (cluster 10) which doesn't overlap with ADHD or OCD or typicality.
I'd also recommend reading https://link.springer.com/article/10.1007/s40489-016-0085-x
[0] See https://doi.org/10.1002/aur.1832 in particular the authors' claim that "our review of research has led us to conclude that using the ASD diagnosis in basic and translational research cannot provide valid conclusions because the diagnosis lacks both biological and construct validity" and also "The ASD diagnosis lacks boundary construct validity..."