Let's not forget the sheer number of advantages the Sith had.
1. 100s of years to prepare, VS. Republic's several weeks
2. Element of surprise, we never expected it
3. Jedi and Soldiers spent several generations without a major war
4. Betrayal of Republic allies
5. Mandalorian Assistance
And they managed to only take 1/2 of the Galaxy before we stopped them dead in their tracks.
Now imagine what would happen WITHOUT all those advantages...
wel before you critise too much, lets look at it the other way.
1: End last major war, we where vicitims of ethinic cleasing. So not much of base pop to start with
2: Limted reasorces to begin with, as we only had a handfull of worlds
3:Limted actual battle expriance, other wise we showen our hand too soon.
4: Having to use our limted reasorces to get allies to turn on the republic
5: Keeping the mandolrians sweet was expensive and time consuming
and with them problems, we still took half the galaxy of you.