The phrase "are difficult to characterise" is correct and usable in written English.
It can be used when discussing subjects or topics that are complex or not easily defined.
Example: "The behaviors of certain species in their natural habitat are difficult to characterise due to their unpredictable nature."
Alternatives: "are hard to define" or "are challenging to describe."
The ionic transport properties of solid polymer electrolytes can limit the performance of lithium batteries and are difficult to characterise.
Wood biomass powders are difficult to characterise for explosibility through standard methods because of their bulky and fibrous nature.
The dynamics of systems are difficult to characterise, therefore, knowledge of the molecular features governing the host guest equilibrium might aid in the design and practical application of the resulting inclusion complexes.
Poor correlations between 3H activities and catchment area, drainage density, mean slope, distance to stream, and landuse, imply that mean transit times are controlled by a variety of factors including the hydraulic properties of the soils and aquifers that are difficult to characterise spatially.
Consequently, morphological intermediates are difficult to characterise.
These cells are difficult to characterise in mammals because of their developmentally controlled migration and their inaccessibility.
But it also finds behaviour "which is difficult to characterise as rational".
The three parts, Heat, Light and Water, involve the past, present and future, and it is the way they blend that creates the sensibility of this work of fiction, which is difficult to characterise.
That's probably the modal estimate among forecasters, and I would say that it is difficult to characterise an economy which is two years out from full employment as facing a tight labour market.2) Having said that, we are approaching the point in the business cycle at which the Fed would historically begin tightening policy.
Some of these approaches result in glazing systems with relatively complex structures and it is difficult to characterise their optical and thermal properties for use in building simulation.
Existing predictive techniques for the analysis of composite structures and components near and beyond their ultimate strength are either based on simple scalar stress functions, or use very complex damage formulations with many material constants, some of which may be difficult to characterise.
