With the caveat that we are only looking at 2 aircraft flying 2 daily rotations between Doha-Frankfurt, it seems that dispatch reliability for the initial period is around 98%.
There has been only 2 issues impacting the operational A350 so far: a hydraulic hose that broke some time ago and a high lift issue that needed a spare part to be flown in beginning of April.
It seems that the A350 is not beset by the myriad of system software issues that plagued the initial operation of A380 and 787.
Source: Tomasz Szykulski
The A350 was effectively a further refinement of the A380 systems and Airbus has therefore been able to focus on system maturity before delivery instead of the exhaustive debugging of functionality that plagued Boeing for 787 and Airbus for A380.
Based on the article “Bjorn’s Corner: Boeing’s 787 and Airbus’ 350 programs, a snapshot” published in Leeham News.