There have also been critiques of diamondoid nanomachines. For example from Richard Jones author of Soft Machines: Nanotechnology and Life.

In any case it is not like we need mechanical diamondoid arms to fix cells. Cells recycle individual molecular machines and organic molecular machines are capable enough to edit genes and fix dna.

Besides outside the brain you can carry wholesale cellular replacement and even wholesale tissue and organ replacement.

Also the diamondoid machines are likely highly susceptible to some types of radiation. A cosmic ray dislodging an atom will like gum up the gears. In space which is a high radiation environment the diamondoids are likely to breakdown by the millions.

I think they could work on specialized vacuum environment but like Jones I also suspect theyd have problems in environments like inside the human body.

In any case it is likely unevolvable molecular machines through advanced synthetic biology are just as capable if not even more capable than the theorized diamondoid machines.