I am pretty sure conquering Constantinople puts the Umayyads in an even worse place than OTL.There is another major consequence to this besides the Muslim world likely being quite a bit larger. The Umayyad dynasty would have its legitimacy basically secure for generations to come, which means that total political unity lasts longer as well.
- The Persians are still going to be resentful of Arab overlords
- Pro-Alid missionaries are having massive success across their empire, especially among malcontents
- Speaking of malcontents they've just added millions of angry Greeks to their list of problems to deal with
- Many of those Greeks are going to be joining up with Byzantine successor states to launch insurrections in the mountains of Anatolia with the terrain and supplies to keep fighting for decades
- Also on that list of problems are the Bulgar and Khazar raiders who will surely be taking advantage of the chaos in Greece and Anatolia
- To cap it all off, at the time of the POD the 'Plague of the Honorable' (716-718ish) is raging in Iraq and Syria