Re: "trying to fly on one engine would make the aircraft very difficult to control or land safely."
It's perfectly possible to control and land pretty much any (propeller or otherwise) aircraft on zero or more engines working. But it is true that e.g. a traditional constant-speed propeller can be tricky when loss of power occurs in that it tries to maintain a fixed RPM by varying the pitch and that means wind-milling (and overwhelming drag with dramatic consequences for glide ratio). Unless you notice what happens and take corrective action by "feathering" the propeller. (Or three.)
But then I would assume that modern ECUs are slightly different animals and possibly don't require quite the same amount of intervention as traditional constant-speed propellers do as far as loss of power and feathering is concerned, leaving you only with the minor inconvenience of loss of power and possibly slightly limited range of (still perfectly controlled) flight. But then again we're talking about a buggy/failed ECU here so who knows. (Well the accident investigators do or at least will know but you know what I mean.)
Now while it is also possible to completely lose control of the aircraft if you fail to maintain sufficient airspeed in an asymmetric thrust situation e.g. during take-off and initial climb I wouldn't expect that to happen to Airbus test pilots (with them being, well, test pilots, and engine failure and asymmetric power being some of the things even airline pilots routinely train for.)
So no, loss of power on 3 engines does not make the aircraft very difficult to control or land, it just may slightly limit the range of the flight and that may in turn limit your choice on the place to land, sometimes leaving you with only sub-optimal options available. Which is what I hope happened here (the alternative being loss of control caused by pilot error when handling the loss of power).