I bet no-one of the people here that recommends doing that radius first actually tried that. If they did they would knew about the problems. It works for simple inlays like dots but for nothing which is more complex or bigger in size. If you radius first you can't even lay your inlay on the fingerboard to draw a precise outline of the inlay on the board. Even if you would manage do get the outline on problems will follow:
1. You can't precisely guide the router base on a radiused surface.
2. The cavities bottom's will have the radius too, but your shell piece is flat
3. If you use scrapwood under the routerbase like setch told it solves the radius problem, but this setup will be very shakey and probably not produce clean results.
I can only state again: Every book on guitar building I read recommends to first do the inlays. Every luthier I heard of does it that way. Doing tight and perfect inlay cavities is difficult enough on a flat surface. There is no disadvantage in doing it that way. So why would anyone want to radius the board first is beyond me....