First of all, it's easier to defend than to attack the city,especially, if it's a fortified city such as Orgrimmar. I think,that Sylvanas knew about this attack and she was prepared for it - just like she was prepared to defend the Undercity. Even if Saurfang knew the weaknesses of it's defense,I highly doubt,that Sylvanas didn't know about these spots too and wouldn't prepare traps in there. Just imagine it's a siege of Undercity all over again - she has blight throwers, troopers prepared to spread it,she is capable to raise the fallen troops as undead under her control. Not even talking,that there would be dark rangers and assasins at every corner,ready to ambush the intruders.
On the other hand,we see a combined force of the survivors of war and a few Horde soldiers - it's clear,that very few joined Saurfang's cause. Also, as Anduin says himself, the casualties of the Alliance are so heavy,that they have enough soldiers only for the last push. Sounds like another contradicting bullshit, cause at the end of the battle for Zuldazar, the army of the Alliance is way stronger and victory is within it's grasp - the only explanation I see here,is that they have lost their fleet to Azshara,thus they suffered have losses in Nazjatar.
I think,that everything is written in a way,that if Saurfang wouldn't sacrifice himself and trigger the Sylvanas to show her true intentions and views,the Alliance and the rebels would most likely fail. It just seems,that Anduin simply doesn't learn from his mistakes - yet again he is attacking the Horde and I didn't see a single soldier with a gas mask on his face. I guess some people never learn. And,after all,if the things would be dire,I think,that Sylvanas would rather blight the Orgrimmar rather than raise a white flag.