The phrase "are very difficult to evaluate" is correct and usable in written English.
You can use it when discussing the challenges or complexities involved in assessing or measuring something.
Example: "The results of the experiment are very difficult to evaluate due to the numerous variables involved."
Alternatives: "are quite challenging to assess" or "are hard to measure."
These common rock mixtures, known as bimrocks (block-in-matrix-rocks) or bimsoils (when the matrix material is soil-like) are very difficult to evaluate.
As head tilting may occur at the acquisition period, differences in the specific shape of the cortex are very difficult to evaluate and are not of use in the segmentation of the rostral temporal lobe.
In addition, a complete evaluation of sexual behavior includes knowledge, attitudes, risk behaviors and more, all of which are very difficult to evaluate because individuals tend to deny involvement in socially undesirable behaviors to avoid stigmatization [ 3].
"These isolated things are going to be very difficult to evaluate," said Carolyn Konheim, a transportation consultant who worked on an alternative, broader plan that was presented to the Department of Transportation.
Integrated information on sustainable development of a company is very essential for decision-making since it is very difficult to evaluate the performance of the company on the ground of too many indicators.
Integrated information on sustainable development of a hotel is very essential for decision-making since it is very difficult to evaluate the performance of the company on the ground of too many indicators.
Liver transplantation might be one of the treatments of choice in such cases, but it is very difficult to evaluate the immune status of these patients due to ART, anti-HCV treatment, and HIV-related immunocompromised state.
The ultimate impacts in all themes were very difficult to evaluate because of the long-term perspective (10 years or more).
It can also be very difficult to evaluate non-uniformity visually since the dimensions of tumours are only on the order of a few millimetres in these rodent models of human tumours.
