I currently have a Specialized Pilot with Shimano Ultegra 6600 Flightdeck Triple Shifter Set on dropdown handlebars. Would it be possible (and relatively easy) to swap out the dropdowns for Butterfly handle bars? Are there any darwbacks or things to lookout for?

Thanks
Rmagers

There's no obvious place on butterfly AKA trekking bars where the drop bar levers would mount "naturally". At best you'd have them splayed out pretty much horizontal, whicn probably isn't a comfortable angle to use them in. So you're probably looking at both new shifters and brake levers. Any short pull brake lever would probably do, then it's just to figure out what you want to do for shifters.
Maybe bar-end shifters + Paul Thumbies shifter pods.

Or a set of "road" flat bar shifters.

It is possible to set up Trekking bars like mustache bars.

IE: locate the brifters on the front of the figure 8 bend,

but the ergonomics of the lever is not, I sense, as well suited
to up-down motion, as it is to sideways finger tip movement..

Straight bar Brifters are a EZ swap over, mounted on the rear, open end .. grips/tape stay on
since the controls are the last parts slid on.

I have my butterfly/trekking bars on 2 bikes with a Rohloff grip shifters
and 1 Magura Brakes, and, 1, V brakes..

Thanks for the feed back. I was thinking about trying the brifters horizontal on the open end, but will now look into the straightbar brifter.

IME drop bar levers require a bend to mount up "properly". YMMV though.

+1, curve in lever needs a bar shaped to curve away from it , or it hits the handlebar,
before adequate braking happens.

look for separate shift/brake levers unless your using V brakes ,
as current MTB brifters
are predominantly long cable pull set-ups.

Put the bars on and go around the block, I thought I wanted moustache bars but one short ride and I knew they were not for me.