Are fna's - even w/ flow cytology - useless for diagnosing lymphoma?

Pretty much. Accurate histopathologic evaluation of lymphomas requires a tissue biopsy, preferably an intact lymph node. Fnas suggesting the presence of a lymphoma should be followed up with a definitive tissue biopsy. Some centers use fna as an initial screening test to determine if the patient's lymphadenopathy is a benign reactive form. The accurate diagnosis of lymphoma based on fna is not possible.
Yes. For initial diagnosis, they are nearly useless. While one may be able to tell a blood cancer versus another type, there are about 150 different types of blood cancers/neoplasms and thus we need more tissue for full evaluation.