The Piaggio will often be found cruising above the citation in a bigger cabin that's quieter on less fuel with no loss in time enroute. The cabin is low, and unlike many turbojets, won't bump or depressurize when the power is pulled back for a descent...and it can be pulled to idle all the way down without losing the cabin...something you can't always do on some turbojets. Ten thousand foot cabin? Hardly.
Quiet...pilots in the Avanti are cautioned to be careful what they say...because the passengers can easily hear them.
Worrying about the boots? Electric forward wing, and bleed for the main wing. The only boots are on the nacelle inlets, which unlike a turbojet engine, are self-separating and don't have nearly the concerns with ice ingestion that most turbojets face.
Furthermore, the Avanti rides turbulence much better with a higher wing loading. Often when other pilots are reporting moderate turbulence, it's barely noticable in the Piaggio.
Let's see...quieter, roomier, less expensive to operate, more comfortable, flies as high and often higher, often faster (but then, what's not faster than a slotation?)...So much for your point of view.
My best guess is that the airplane has its drawbacks in following fields:
Runway required
Range
Baggage space and how you get the stuff in (if that is relavant)
Behaviour on the ground (taxi and crosswinds - but that is just hearsay and certainly managable - but apperantly a bit of experience is big plus then)
Pistol refueling.
FAR 23 - if you are interested in FAR 25 T/O performance.
Runway required...it's got turbojet performance; plan on the same runway values that will accomodate a turbojet. It will do less, but given acceptable accelerate stop and go distances, or balance your field, and you'll plan as though it were a jet. Runway distance isn't a problem.
Range...plan on four hours, with a still-air generic TAS of 370+. In the US, eastbound, I've easily done coast to coast non-stop. You won't get that out of a King Air.
External baggage, and it holds more than you might think. I've hauled skiers into Aspen and Telluride on many occasions in the Avanti, with an airplane full of people, bags, and their skis. Not a problem.
I don't know what behavior on the ground others might consider to be a problem, but I've taken off and landed in very stiff crosswinds without difficulty. I've operated it from iced-over runways, from snow, from water covered runways...just not a problem. I've landed at high elevations and low, in gusty conditions and calm...it handles very well.
I have no idea what "pistol refueling" means, but if you are referring to hand fueling via the upper tank refill...it's not necessary. The airplane comes with single point refueling capability and automatic shutoff, and it works very well. Sounds like you were misinformed.
Yes, it's a Part 23 airplane. However, particularly in comparison to other turboprops, it performs very well, and I always planned my flights with full accelerate-stop and go protection, and the ability to meet the climb gradients applicable to the location and procedure in use...with an engine out. Single engine performance doesn't decrease significantly as density altitude increases; it still performs about the same. Unlike some turboprops, the airplane can and will fly a single engine missed approach.
It's flight characteristics are very user friendly, as are it's systems. Stall behavior is very benign, it handles well throughout it's operating range.
Rather than going with heresay, perhaps you should have taken a demo flight and seen for yourself.
I did a lot of demo flights for prospective buyers or investors, and flew pilots and non-pilots alike. I always printed all my TOLD performance data on the weight and balance form so the passengers could see for themselves if they so desired; nothing hidden, no chances taken, everything done safely...and the numbers always worked.
Certainly the Piaggio does have it's disadvantages, but most of those which have been named just aren't the case, or are mis-stated.