^This. Remember Bail Organa. "We need weapons and starships and people with the courage to use them."
A rebellion like the Rebel Alliance to Restore the Republic would never have risen without Vader and Sidious. The "leaders" were too scared to do anything real.
You fail to take into account however, that it would be the SE supplying them with weapons, starships and people - they would basically do exactly the same thing that happened in TFU (except this time they would be doing it deliberately, rather than accidentally) e.g. the Separatist Movement on Ord Mantell was being supplied by the SE, just as the Balmorran Resistance on Balmorra was being supplied by the Republic.
Oh and question. I'm pretty sure all the Empire did was gather all the leaders in one place, so they could be wiped out in a single blow. They didn't supply them with weapons and starships - that would be beyond stupid, surely?
I usually like to support the underdogs in these threads, because then I have to convince myself as well as others.
That being said, I do think the Sith Empire is the underdog here. The Galactic Empire is just so.... HUGE. (Quantity over quality, anyone?)
The Kaggath comes down to the death of the person. Be the military strong or not, if there's infighting or not, there still has to be a plan to take down the leader.
I just don't see how the Sith Empire could kill Darth Sidious.
Endgame for both sides has been discussed. In a nutshell it involves the sacking of Dromund Kaas or Coruscant - and the success or failure of such a move depends on how successful either side is in the war. If the GE are winning, they will not be ambushed on Dromund Kaas by Harrowers (it will also be easier for them to discover the coordinates) while they are looking for the Emperor's invisible fortress. Which they can blast to pieces unhindered. They will also need a sizable fleet to invade the planet and overcome the initial forces which depends on advantages etc etc
And the same goes for Coruscant. They will need to be winning the war so the GE forces are spread thin, so they can easily blast a whole through the SDs guarding the planet and orbitally bombard the imperial palace. If they are not winning the war they'd likely have to contend with a SSD and reinforcments which would no doubt be lying in ambush (due to Sidious' foresight). But if they are winning the war the intial force will be inconsequential as will the reinforcments which will be minimal. And if they are winning the Ascendant Spear will likely be still operational and will strike faster than expected, allowing them to bombard the palace (and kill the Emperor) before reinforcements can arrive. Then there's the chance Sidious could escape, either via the Lusankya (unlikely) or his personal Sith Interdictor (likely). The success or failure of such a move also depending on the status of the war (not as much though) as the SE would have stolen the plans, or simply stolen, one or several interdictors as it was technology they did not possess. Much like Angral stole the plans to various super weapons.
So as you can see both combatants are perfectly capable of being killed, it just depends on who wins the war.