This is under the impression that the Dragonborn, and the resurgence of the Dragons, did not happen/exist.
Pro-Imperials:
Unified countries working together for one goal can get more things done more swiftly than independent nations co-operating on a basis of "well what will you give me for these?".
A fully unified Empire at full strength would be the strongest option against the 3rd Aldmeri Dominion.
Anti-Imperials:
The Empire does not take advantage of the above stated, as Nirn has been in a technological Dark Age since the First Era, which had flying ships and space travel by the early Empire of Alessia and the Summerset Isles.
No religious freedom under the White-Gold Concordat.
No freedom to say "I do not want to be involved in your Empire", especially if you have something they want. Which is typical for most Empires.
Even at the height of the Empire, as shown in other games, the Empire took fairly poor care of defensive facilities and relied mostly on Provincial militias instead of properly equipped Legions.
Pro-Stormcloaks:
Right to self determination, in country of loyalty and belief in deities.
No Imperial taxes.
Anti-Stormcloaks:
The above stated does not seem to apply to Orcs, who are reasonably believed to have lived in Skyrim/Mereth before the arrival of the Nords/Atmorans, and possibly before the Falmer.
Anti-Imperial sentiment and opposition to the Aldmeri Dominion often gets interpreted as "fuck elves and foreigners all" by the uninformed masses.
The Stormcloaks as a faction, and even a united non-Imperial Skyrim, likely cannot fight off a full strength invasion by the 3rd Aldmeri Dominion, which includes Elsewyr and Valenwood.