The phrase "are very difficult to compare" is correct and usable in written English.
It can be used when discussing items, concepts, or situations that present challenges in making comparisons due to their complexity or differing characteristics.
Example: "The two theories are very difficult to compare because they operate under different assumptions and frameworks."
Alternatives: "are hard to evaluate" or "are challenging to contrast".
The reported costs for the two projects are very difficult to compare since the situations were so different.
Because its forms and outcomes are so varied, studies are very difficult to compare.
Due to these differences in the use of SOFA scores and in the methodological approach and quality, results of individual studies are very difficult to compare and meta-analyse.
Through variation in glucose monitoring techniques, the choice of clamp glucose thresholds, the definition of end of action, the timing of injections in relation to the start of the clamp and the differing protocols with respect to the use of insulin infusion to switch off hepatic gluconeogenesis, published clamp studies are very difficult to compare relative to each other.
These links between soil chemistry, farming practices and the characteristics of size spectra indicate the possibility of assessing the status of soil ecosystems, even across large geographical ranges and soil types that are very difficult to compare using more traditional taxonomic indicators.
It's very difficult to compare them in that way".
But it is very difficult to compare students academically before they have undertaken GCSEs.
John Barrett, a BBC commentator for the past 33 years, says: "It is very difficult to compare players from different eras because conditions and equipment change.
For all we know, we could be rubbish in Barnsley, because it's very difficult to compare ourselves with other hospices that don't publish their data", said Carey.
I have long believed that GDP understates growth even in rich countries, where its measurement is quite sophisticated, because it is very difficult to compare the value of baskets of goods across different time periods.
It is very difficult to compare wealth inequality estimates from the UK Wealth and Asset Survey with estimates for other countries based on estate returns from the equivalent of HMRC in other countries, because we know for the UK that WAS gives a much lower estimate of wealth inequality than the estate multiplier method using HMRC inheritance tax returns.
