Generally there are a couple of scenarios that would need to be considered when evaluating the medium of a message.
1) Capability of the medium to convey the message.
2) Capability of the audience to receive that medium.
3) Contextual issues with the medium.
1) For example, you wouldn't want to use a sign if the message was related to something auditory or that had a relevant scent. There are things that signs can't display very well including moving graphics, sound related messages, etc.
2) Or, if the population expected to need the sign wasn't able to process it because they were blind or illiterate.
3) Signs are not very effective in highly visually distracting areas by their nature. So areas that require high levels of concentration (complex machinery, fast driving, flight) or that have lots of extraneous imagery (think Times Square's flashing lights) would mitigate the ability of the sign and the audience to translate the message.
Hope this helps, please feel free to reach out to me if you have any additional questions or concerns.