The phrase "are frequent and complex" is correct and usable in written English.
It can be used to describe occurrences or situations that happen often and are intricate in nature.
Example: "The challenges we face in this project are frequent and complex, requiring careful consideration and strategic planning."
Alternatives: "occur regularly and are intricate" or "happen often and are complicated."
Because the transfers among the modes are frequent and complex, we adopt the Controlled Petri Net to design the external events and condition of its operating modes.
Handovers of care in hospitals are frequent and complex due to multiple healthcare professional shift changes each day, inter-hospital referrals, and transitions of the patient from one care setting within the hospital to another (eg, from the emergency department to the operating room or the inpatient ward).
Gangland killings are frequent, and go unpunished.
The battles were frequent and legion.
The criteria, he says, for judging whether a child needs professional support are whether the hallucinations are "frequent, complex, distressing and cause impairment".
Comparative genomic hybridization arrays and fluorescence in situ hybridization (FISH) studies have shown that HER2-amplified breast cancers present a variety of alterations on Chr17 and gene amplifications at Chr17q12-q21 are particularly frequent and complex [ 5].
Anatomical and genetic abnormality in patients with ADPKD may be more frequent and complex than previously believed.
We conclude that the anatomical and genetic abnormality in patients with ADPKD could be more frequent and complex than previously believed.
Patients with COPD, who are frequent in family practice, have a complex profile and pose a clinical and organisational challenge to FPs.
There are frequent low ridges and hills.
Rhythmic structures in this area are highly complex; there are frequent changes of metre, various durational values, and intentional tempo displacements between the singers and the drum.
