Jane,
No reason that I know about.
It can be a little difficult because the brass is often not much harder than the silver or copper.
It can be be a little tricky to get a punched up undercut. But you can just undercut with a knife graver in that case or even scrape in an undercut with a scribe.
Best of luck, please show it when it's done.
i have inlaid copper into nickel with no problem. the inlay was somewhat proud of the surface. that was by design. i didn't know what i was doing, but it worked !!
Shouldn't be a problem, if you're careful and don't mess up the brass. I was foolish enough to inlay hard silver solder into soft copper. Even that worked out allright - http://www.knivesandengraving.com/2008/10/silver-inlay/