Although I'll say this for the Legion. They need to become a little more aggressive and assertive... that will take time and prob is not going to happen until the Empire can prove itself again. They're not bad, it's just they need to adopt a more vicious, not gonna wait for you to kill me in my sleep attitude.
Kinda like shoot first ask questions later.
Tullius' Auxiliaries are another story though. The Legion can be aggressive and in the past have been extremely aggressive in war. Skyrim isn't a very good example on the Legions, since they're not the proper Legionnaires but a Militia. Also they're not invading, and Tullius is fighting two battles. Political and Military, if he could just replace the Jarls with his Legates the war would be over more quickly.
The Empire did not have that strong a grip in Summerset Isle anyway... or Valenwood apparently.
The Empire didn't have a grip on anywhere. It's how they lost Morrowind, Valenwood, Elsweyr, Summerset Isle, Black Marsh. They weren't abandoned by the Empire, the Empire merely lost the grip over them and released their grasp to recover more closer to home.
I don't fault the Empire for releasing the provinces they did. It's always easy to say the Empire abandons you the second it can, but it wasn't a choice they did lightly. The destruction the Oblivion Crisis brought was total loss of Imperial authority. The Empire had to cut losses, stabilize then it could deal with other matters.
The provinces took advantage of the Empire's weakened state.
While Ocato was a good man, he was a reluctant ruler. The Empire needed a strong Emperor, the Mede Dynasty was a blessing. It brought them a ruler that was respected by the Legions, respected by Skyrim who endorsed him as their Emperor. He also took the Empire from the hands of Emperor Thules the Gibbering, which many people forget when they talk about the Mede Dynasty. That Mede was a lot better than the Emperor that had claimed the throne, luckily Mede took it from him.
'Without Titus Mede, there would not be an Empire today.' - Rising Threat, Vol. IV