Slides - IIRC there was a good discussion about the issues way back when they introduced the 747 and Tripp of PanAm was worried about how people would get off
If you make them very stiff they are heavy (bad for economics) and can be dangerous to both the passengers, the fire crews and the airframe. - there is also a problem in fitting and deploying a "hard" slide.
And if you want them to double as temporary life rafts...................