BEWARE - THESE ARE CLEARLY PERSONAL OPINIONS, MANY OF WHICH ARE OUT-DATED, OUTMODED, AND FACTUALLY NOW IN-CORRECT.

1) they are 'trendy', when the trend goes you have lots of holes in your ceiling (EVERY TYPE OF LIGHT FITTING COULD BE DESCRIBED AS 'TRENDY')
2)the rest of us use energy saving bulbs to save money/the earth, where as converting to downlighters uses a lot more electricity/ money THERE ARE PERFECTLY ACCEPTABLE ENERGY SAVING DOWNLIGHT FITTINGS AVAILABLE TODAY.
3)if not installed right they can cause a fire TRUE - AS WITH ANY ELECTRICAL INSTALLATION
4)if you install them right (and remove loft insulation from around them) then your ceiling will not be as well insulated any more, whilst the rest of us are putting more insulation on to save money/ the earth.
5) the light they give is poor, high in some places low in others, lots of shadows etc. WHEN THE OVERALL LIGHTING DESIGN IS POORLY CONSIDERED. WHEN PART OF A WHOLLISTIC LIGHTING DESIGN, THEY WORK VERY WELL. THAT'S WHAT ARCHITECTS AND LIGHTING DESIGNERS DO FOR A LIVING
6) they break the fire integrity of the ceiling (only legally an issue in flats and garages, but still something to think about)TRUE, BUT YOU CAN GET PROPRIETARY FIRE HOODS FOR THESE FITTINGS
7) they allow sound to travel through the ceiling more (again more of an issue when the room above is inhabited)
8) you need to place them carefully according to your furniture layout, if you move your furniture they are then in the wrong place.
AS IS ANY CEILING MOUNTED LIGHT / LAMP / PENDANT 9)the government are proposing to ban incandescent bulbs fairly shortly, LED replacements aren't up to scratch and CFL's aren't too good for downlights either as they stick out 10-20mm (NOT NOW THEY DONT)
10)drops of very hot metal and glass from an exploded lamp Link
11) the bulbs blow more often especially when there are voltage fluctuations, and they are more expensive to replace. YES, THEY ARE PRICY