The phrase "a complex one that" is correct and usable in written English.
It can be used when referring to a situation, problem, or concept that is intricate or multifaceted, often followed by an explanation or elaboration.
Example: "The issue at hand is a complex one that requires careful consideration and analysis before making a decision."
Alternatives: "a complicated matter that" or "an intricate issue that".
The decision of a surgeon to operate and of a patient to accept an operation is a complex one that goes beyond objectivity on the part of both parties (Lurie et al. 2008, Deyo 2009).
The question of whether breaches should be a criminal offence is a complex one that requires more work, especially on how this would be enforced.
The relationship between disability and cognition is likely a complex one, that is currently incompletely described in the literature.
The decision to undertake systemic treatment of psoriasis is a complex one that requires both experience and judgment.
Such programs have real benefits for families, but the alliance between drug companies and patients is a complex one that can make ethicists and health-policy specialists uneasy.
The Russia-Ukraine-Crimea story is a complex one that requires a news consumer to engage deeply in the subject matter.
Competitors will take a simple concept and develop a more complex one that serves a better purpose.
This program determines the simplest model of evolution that cannot be rejected in favour of a more complex one that gives a significantly better tree.
A selection of the most predictive questions by CART analysis provided a stronger algorithm, though a more complex one that was far from perfect.
The answer is an unnecessarily complex one that dates to a time when fighting fires was still the main mission of most fire departments.
His father had recently gotten a promotion, or so I think I was told, so his family had moved to a newer apartment complex, one that was at least a ten-minute walk away and therefore off limits.
