I looked at it myself a while ago...The HSF is not a health insurance. Many people who are on those plans seem to combine them with a traditional private health insurance. It seems to be good for medical day to day expenses.
For example, if you are admitted in hospital, I believe that they pay a set fee per night, depending on the plan you are on. They will not pay for inpatient fees, surgery etc...
Many expenses are not covered,
You must also claim within 6 months of incurring the expense.
The recurring statement seems to be that it does not replace a private health insurance.