The spacing between cogs in different – think 9 campy cogs/spacers is actually wider than the Shimano equivalent. If you want to see if you could live with it, give the adjuster on your mtb rear mech a couple of turns and see how long before you go mad with the grating of cogs just slightly off from the chain….
Its a small difference but enough on such a delicate setup to be annoying at the least.
Having said that I’ve run a Shimano cassette on an otherwise Campy road bike for a while before I went the whole hog – the low gear would slip a bit and I’d always have a tiny (maybe imaginary) feeling the other cogs just weren’t meshing properly. There are various tweaks you can do – can’t remember the websites.