The phrase "remains difficult to ascertain" is correct and usable in written English
It can be used when discussing something that is still hard to determine or understand.
Example: "The exact cause of the phenomenon remains difficult to ascertain despite extensive research."
Alternatives: "is still hard to determine" or "continues to be unclear."
Although the state of the economy remains difficult to ascertain beyond dubious official figures, huge sums are lost to corruption.
According to the People's Movement Against Nuclear Energy (PMANE), 302 women and 24 men are engaged in a "fast until death" hunger strike, but it remains difficult to ascertain with any degree of authority how many of these men and women are in fact supplementing their diet in some way for the preservation of health and safety and to what degree that might be the case.
However, it remains difficult to ascertain whether it is the aging process or the cancer treatment that produces functional decline.
A monetary estimate on a national, state, or hospital level of the financial impact of PRWORA remains difficult to ascertain due to data limitations.
Despite the improvements in VTE risk assessment and prevention discussed above, it remains difficult to ascertain with certainty if monetary incentives were the driving force behind the significant changes in the percentages of patients risk assessed between 2009 and 2010 in each hospital.
Manifestations may range from increased fatigue to overt heart failure, but due to the underlying disease and the high rate of co-morbidity in the elderly patients, it remains difficult to ascertain the contribution of the various etiologies to the respective symptom.
In the absence of routine active surveillance (e.g., posttransplant testing of all recipients to ascertain transmission events), donor-derived infectious disease transmission remains difficult to recognize and document.
So the choice remains difficult to explain.
The legal landscape remains difficult to navigate.
It remains difficult to predict their future".
However it remains difficult to assess objectively.
