It's not easy being a portable cooling unit. Since these units are often moved from place to place, they can be jostled and bumped around on the back of a truck before being unloaded at an unknown destination.
At any given location, power supplies may be inadequate, causing problems with equipment operation. Or, the individuals setting up the equipment may not be familiar with what the equipment can tolerate. The latter can result in flex ducts either being too long for the external static pressure capability or choking off the return and supply air.