Firstly, the loss of an engine and a hydraulic problem could very well be chalk and cheese when compared to each other. To compare the two and then direct criticism to a crew for not declaring an emergency for a problem which is purely stated as a 'Hydraulic' problem is shortsighted.
ATCOs need to be aware that aircraft are designed with a significant amount of redundancy in many systems nowadays so what may seem like a crisis to an ATCO could purely be viewed as a niggle by the Flight Crew. I appreciate that ATC need to make a mental model of the situation (possibly difficult with some foreign operators) but jumping to conclusions based on past experiences could lead you up the wrong garden path. The element of trust has been eroded - you are trained to control and we are trained to fly. The two tasks are very different and both require specalised training and that training is in depth for the aircraft type that we are trained on. I would advise you not to speculate on what flight crew may be faced with or what flight crew are doing since you could well end up grabbing the wrong end of the stick.
I have no hesitation in declaring a PAN or a MAYDAY - providing the situation merits it. However with the redundancy built into many aircraft systems nowadays - in some cases it could take multiple failures of the same system for the situation to 'concern the safety of the aircraft', which after all is the rough definition of a PAN! So just because some one mentions hydraulics (or any other systems failure) on the R/T does not suddenly signify that the safety of the aircraft is brought into question.