I will be flying home to the states from Italy with my bike in a cardboard box, unable to find a hard case out here in Italy. After some internet research it seems some people prefer to fly with the cardboard box because it "needs" to be upright, and baggage handlers are less likely to stack things on top of it… lets hope thats true! Wish me luck…
If you are counting on the baggage handlers to adhere to the "fragile" or "this end up" marking on the box, you are going to be sadly disappointed. Bike boxes won't fit through the conveyor system in the upright position. I'd be willing to bet a case of your favorite beer that the box will come out of the oversized baggage handling chute on its side. I have seen this too many times to count. And I have seen bike boxes at the bottom of the baggage carts, buried in suitcases waiting to be loaded on a plane. I literally watched my bike fall from the airplane cargo hatch to the ground because the baggage handlers were sloppy.
If you must use a cardboard bike box, try and use a box in a box. If you have a removable derailleur hanger, remove it and the derailleur. Put dropout protectors on the frame and fork dropouts. Buy some foam pipe insulation and put it over all the frame tubes and fork legs. Zip-tie all parts to the frame so nothing rattles around loosely. Put small parts in a sealed padded envelope or smaller box within the bike box. Fill the gaps in the box with packing paper or foam to prevent things moving around. Put as much Styrofoam onto the box corners as you can while still fitting the bike. Then strengthen those corners (inside and out) with fiberglass-reinforced packing tape.
Your bike can survive a trip in a cardboard box. But it is not ideal. All it takes is one delay and your cardboard box is sitting outside on the tarmac in the rain or, worse, next to a plane being hosed with de-icer. Your cardboard box them becomes a bowl of oatmeal with a bike inside.