They are powerful enough to be a threat, but looking at the events around the Great War, I think their strength is often overestimated. They took a major blow to their military might in Cyrodiil during the Great War and they were unable to keep the southern parts of Hammerfell, even when a lot of the military forces had been sent to Cyrodiil and the Redguards had internal strife up until the moment they were invaded. And this is one of their neighboring nations, making supply lines a minor problem compared to what it would be to invade Skyrim or High Rock at this point.