Complex sentence: Metagenomic sequencing could prove especially useful for diagnosis when the patient is immunocompromised: in such cases an ever wider array of infectious agents can cause serious harm, and diagnostic methods that rely on the detection of antibodies are more likely to fail.
Split into simpler sentences: Metagenomic sequencing could prove especially useful for diagnosis when the patient is immunocompromised. Third, diagnostic methods that rely on the detection of antibodies are more likely to fail.