I'm familiar with Shimano, but from what I understand, this applies to Campy and SRAM. With that caveat and an apology for going over stuff that I suspect you're already fully aware but presented for clarity ...

A front derailer -- say Shimano 105 -- will have a shape appropriate for the chainrings in that group. Last I checked, a Shimano 105 triple has 50-39-30 tooth chainrings with ramps and pins. Consequently, the outside of a Shimano 105 front derailer will have a shape that mimics the 50-tooth chainring while the inner expects a 39. STI needs this optimal setting for crisp shifting. Put a 52- or 53-tooth chainring in there with the 105 front derailer ... something a bit out of spec with ramps and pins ... and STI will still work, IME. But it will get a little sloppy. As you get further out of spec or lose the ramps/pins, STI performance quickly degrades.

However, if I were to friction shift the front derailer with say a bar end, I can still get pretty good shifting at combinations where STI chokes.

My limited experience with Campy where the front shifter has many micro positions suggests that it would be more robust to off-spec combinations. I've never ridden an SRAM brifter.