Neoload allows for changing the user-agent string to mimic different device behaviour. If the site does'nt solely depend on UA string to determine the contents it has to send based on the device/network configuration then how to handle such scenarios ? In that case just changing the UA string won't replicate the different device behaviour.
Consider a case where a site responds with content based on the device resolution. Like a device with a resolution of 360X640 gets images of x Kb and a device with a resolution of 800X1280 gets images of x+y Kb. So in this case they are dependent on the resolution values which are not by default sent over UA string. In this case even though NeoLoad replicates at protocol level it wont be a real device simulation.