whom就是前句中的people with serious illnesses
some of whom have been unable 这个定语从句中,some of whom=some of the people with serious illnesse,
因此=some of the people with serious illnesse have been unable to obtain The health-care law's coverage
因此是做主语,因为有of 介词提前,不能用who,只能使用whom
医保法受到了身患严重疾病患者的欢迎,因为,他们中很多人以前享受不到这些医疗保险福利