The phrase "are somewhat complex" is correct and usable in written English.
It can be used to describe something that has a level of intricacy or difficulty that is not extreme but still notable.
Example: "The instructions for assembling the furniture are somewhat complex, requiring careful attention to detail."
Alternatives: "are fairly complicated" or "are rather intricate."
The terms of the deal are somewhat complex.
But when development and commercialization relationships are somewhat complex, the company typically forms alliances, ranging from stand-alone joint ventures to cross-site collaborative partnerships.
However, since there are a number of process parameters and their relationships are somewhat complex, realization of the process design and quality control is accordingly considered to be difficult.
Given the collocation of D2 and D3 receptors, and generally higher densities of D2, the separation of D2 and D3 information from [11C]--PHNO PET data are somewhat complex.
One reason for this neglect is that the connections between policy and outcome here are somewhat complex and counterintuitive, and therefore difficult to explain to rationally ignorant voters who pay little attention to the details of policy, and may not know even very basic economics.
However, the numerators are somewhat complex.
The influence of LCB level on cell size was somewhat complex.
The method is somewhat complex; it involves the comparison of climate observations with the output of climate models.
The details are intentionally hazy, but the over-all project, as McAfee explains it, is somewhat complex because of its decentralized nature.
The functioning of VAT is somewhat complex, because it involves paying the tax at the point of sale, but also deducing any taxes paid.
The kinetics of curing or vulcanization is somewhat complex as it depends upon the compound formulation, temperature and in some cases pressure.
