Reasons not to Join Japanese National Health (JNH):
1. It does not provide good cover outside of Japan, and the pension portion is confiscated if you leave before contributing for 25 years.
2. The JNH system only covers 70% of your hospital bill, MedExPlan covers 100%.
3. It does not provide the extra benefits that expat policies have like medical evacuation, compassionate travel, home nursing etc.
4. Clinics catering to foreigners with American and British doctors in Tokyo do not accept JNH.
5. Most foreigners prefer not to be treated in a Japanese hospital, given the choice. The average hospital stay in Japan is 34 days while in Europe it is 7, and in the US it is 5 days. (statistics: American Chamber of Commerce Journal article) If you are really sick or need elective surgery, private insurance enables your return home or to a country where they speak your language and hospitals are up to European and US standards of treatment.
6. Medical malpractice: In Japan it seems to be common from reading newspaper reports. You do not have much hope of success in litigation, or of getting a good settlement even if you win. The Japanese legal system does not have juries, the courts proceed glacially, and there is often discrimination against foreigners. If you would be satisfied with an apology, they express regret a lot here as long as they don't have to pay and nobody loses their job or actually has to takes responsibility for incompetence.
Reasons to Join Japanese National Health:
1. If you have a big family, i.e. 4 or more children, JNH can sometimes be cheaper than private expatriate insurance depending on your income.
2. If you join you can get into the unemployment insurance & pension system.
3. If you have pre-existing conditions or chronic diseases like diabetes that may require expensive maintenance, and or flare up seriously from time to time, JNH will cover them. Private insurers like MedExPlan will not if they are pre-existing conditions.