It depends on a lot of things. Is it the 5 yr or 100 yr floodplain? Is there now a dam upstream? What type of facility? What kind of riparian habitat is there? What type of soil?
I'd think the major con to something like an infiltration bed placed in a floodplain, as LCruiser gets at, is that the infiltration of such a facility may be minimal at best, as groundwater in a low area like a floodplain is likely to be very close to the surface, leaving you no depth to actually infiltrate water.
Additionally, putting any sort of embankment in that floodplain area will have an effect on the flood depths, and could change the flood elevations. There's also a question of whether the relevant permitting authority would allow such a facility within the floodplain.
the facility will require significant maintenance, reconstruction after any significant flood event. Generally it is better to place these types of facilities closer to the source - ie: higher up in the watershed