There should be a valid reason (‘Usar’/undue difficulty) for combining and Qasar of Salah, and it is upto the individual to decide whether he or she is faced with such situation or not. Salah cannot be combined without any valid reason. In all circumstances, decision should be honest.

‘Undue Hardship‘ is not defined in Sharia because it is not possible to encompass every scenario an individual may face till the end of time where principle of undue hardship will be applicable. And secondly, the matter has been left to the individual to decide; it is to be decided by the individual where unusual hardship has struck them and whether they want to take advantage of the relaxation or not. The only requirement is that a Muslim should make an honest decision, and if there happens a mistake of judgement on their part, then hopefully Allah will forgive it.

The relaxation here does not deal with basic hardship which is required to conduct certain affairs of Ibadah in normal course of action e.g getting up from bed in winter and doing wuzu etc. the relaxation in sharia deals with the kind of hardship which is over and above this basic level of hardship which is known to every individual e.g. when a person is sick and getting up from bed becomes more difficult than usual.

In my understanding, sometimes the situation in some regions can actually take hardship to a high level where there remains no room but to combine the Salah because of small window of time between two Salah timings and regular sleep timings being impacted unusually. But at the end, an individual has to decide according to his own cirumstances.

Please also refer to the video below from 10:30 to 13:06

https://youtu.be/4vMthgdqYHg?t=630