There are several problems, which may and may not matter to you too much if the intended use is on a turbo-trainer.
Chainline will probably be wrong, as others have noted, the cranks sits further “out” if it’s Campagnolo on Shimano (ISO on JIS), unless the tapers inside the crank are very worn – in which case you may get away with it (but I doubt it). This will have effects on shifting at the front & possibly give you problems on the big-big combination at the rear – the chain might tend to fall off inwards in this gear and maybe the next one, too – depends how worn other bits in the transmission are.
FD might and might not shift – but probably will. Bear in mind the same FDs are made to work with varying different diameters of seat tube so are not at a constant distance on every frame from the crankset anyway. The biggest problem would likely be on a 28.6 seat tube (trad steel) as the FD already has to throw further, without adding in the problem of an incorrect chainline / crankset 4 mm odd further out …
Some possibility that in the long term the LH crank will split because there is not enough load-bearing surface engaged inside.
