The phrase "are frequently complex and" is correct and usable in written English.
It can be used to describe something that often has intricate or complicated characteristics, typically followed by additional information.
Example: "The issues we face in this project are frequently complex and require careful consideration to resolve."
Alternatives: "often intricate and" or "commonly complicated and".
Small organisations often miss out on funding from big donors because they do not have the staff or language skills to access application forms that are frequently complex and bureaucratic.
Relationships between rainfall and landslides are frequently complex and uncertainties tend to be amplified by inconsistent and incomplete landslide catalogues and sparse rainfall data availability.
Kinetic expressions of the enzyme-catalysed reaction rate however, are frequently complex and establishing accurate values of kinetic parameters normally requires a large number of experiments.
Cox models have been used for economic analyses since cost distributions are frequently complex and difficult to describe parametrically.
The scaffolds of natural products are frequently complex and rich in stereocenters, which necessitates the development of efficient enantioselective methodologies.
Apparently 'simple' interventions are frequently complex and unpredictable in their effects (Read et al. 2011; Derde et al. 2014; McVicker et al. 2014; Nicolle 2014).
Please note that the sentence construction in the EYLF is frequently complex and grammatically metaphorical, requiring a degree of interpretation on the part of the analyst (Halliday and Matthiessen 2004).
Accountability is frequently complex and not always reducible to linear relationships that are formed by the understandable need for making recipients accountable for the use of donor funds.
They are estimated to comprise approximately 28% of all surgical wounds and are frequently complex to manage.
Coronary artery lesions in diabetic patients are frequently complex owing to these lesions' small size and diffuse involvement with atherosclerosis.
Databases used for modeling are frequently complex, requiring accurate information on timing, dates, and amounts of the drug administered, sample collection, and associated demographic and laboratory information.
