If you were only focusing on moving patients from compartment A to compartment B on the same floor, then evacuating with a bed might be faster. However, the doorposts should be wide enough to be able to get through the door at all.
The main disadvantage of evacuating with beds is that they can cause blockades in the corridors of the safe compartment. Preparing for a horizontal evacuation alone is often not enough. If you think that a horizontal evacuation will be sufficient, it can lead to a lack of focus in risk assessments on a possible vertical evacuation. This can create gaps in the preparation for large-scale assisted evacuations (e.g. by the fire service) and in the provision of training and evacuation aids to descend the stairs. Evacuation with beds can therefore be faster, but it certainly entails high risks.