I don't think any of us is underestimating the Umayyads,we are just trying to find ways to explain why no power around the Mediterranean,not necessarily the ERE were able to unite the region once again.
I understand, but part of the reason the other states didn't is that they had no claim to Rome nor the empire. Byzantium did and at certain periods reconquered much of the empire and on the eve of the Arab invasion had destroyed its Sassanid rivals making them if need be vassals. This development could easily spur Byzantium back on the conquest of Africa, then to Italy and so forth.
The Mediterranean as well was still a vital trade zone and relatively safe. The possibility of Pax Romana and Marr Nostrum remained in the future of a great king or emperor to grasp. The Umayyads however took these notions and eliminated them by being the first state since Carthage to challenge Rome/Byzantium in the Mediterranean. It further did not simply challenge but conquered and destroyed the trade within the eastern Mediterranean and subjected coastal Europe to onslaughts from the south and the west.
Essentially the Umayyads made the concept of Rome and Pax Romana obsolete, sort of the last nail in the coffin. Interestingly, it was one of the few things the Umayyad actually did, they almost were a state dedicated to waging war on the notions of Rome and of course the Khazars. The Abbasids afterward would come more to define Islam than the Umayyads but the Umayyads defined Europe and Rome more than they did Islam, quite interesting, I must say.