I personally believe that is Caesar had lived beyond 44 BC, his Parthian campaign would have ended in failure. Think about it, Parthia's army was made up of lightly armoured cavalry archers and heavy cavalry. The bulk of Caesar's army on the other hand was made up of slow moving (yet powerful) legoinaries. If these armies were to clash Parthia's cavalry would easily pick off Caesar's men.
Furthermore, Caesar would have been taking on a superpower of the time. Unlike the divided Gallic tribes Caesar was used to fighting, the Parthians were organised, well equiped, rich and more importantly more united than the Gauls. A terrifying and formidiable aponent if you ask me.
If you don't believe me look at how many Roman military disasters there were against parthia:
-CRASSUS 53BC, Legions anialated by Parthia at Carrahe
-DECIDUS SAXA 40BC, Lost Roman standards to Parthia
-MARK ANOTONY 36BC, Was deafeated by the Parthians in Armenia
No wonder Augustus wanted to create a peace with the Parthians in 20 BC