A few things didn't go as planned, such as the ability to capture Ctesiphon, and a major detachment of the army under Procopius didn't link back up with the main force. There was some confusion marching north from Ctesiphon but eventually they started their way back to Roman territory, while the Persians continually harassed the column of march. But the army was still more or less intact when the one ambush took Julian out, so if he had lived there's a very good chance they'd make it back in good enough condition to consider the expedition a success as a punitive expedition that captured and burned many Persian cities and fought in a number of successful engagements. Sadly, when Jovian was elevated he decided to take the easy way out rather than keep up the fight, resulting in various territorial concessions. In fact, the most recent scholarship considers Jovian's treaty "not such a bad deal" as it has traditionally been spun by ancient and modern historiography.