The only way to deem it safe to travel was to listen to the authorities who said it wasn't. The storm, whilst short and not very strong, was extremely widespread causing difficulties all around Manchester and the country. I'm sure the emergency services would have wanted congestion, road fatalities/injuries and stranded members of the public on top of everything else. Yes the game could have gone ahead but would have had to have been behind closed doors.
Comparing it to the Sheff U game is not quite the same. The opposition fans would have been much closer to the ground when the storm was due to hit (later in the day in Sheffield) plus they had some flexibility in delaying KO, not the same with ours, and traveling fans had more time to get back home.
Right decision was made.