I have found the same to be true. Even if you set your policies to something silly like 1 hour, it won't necessary perform an auto-purge.
Another problem is that while you can auto-purge your Health Monitor runs, this only purges records from the v$hm_runs view and leaves associated records in the v$hm_findings and v$hm_recommendations as orphans in those table.
Oracle's only fix, according to an SR I opened, was to 'DROP_HM_SCHEMA' - an undocumented function.
Also, it doesn't housekeep the text versions of the alert and listener logs - you have to manually housekeep those yourself, which is quite ridiculous.
I spent a lot of time over the last couple of weeks on this and I consider it something that's not really ready for prime-time (in the same manner ASMCMD was awful a couple of releases back). Has potential if it can become a 'one-stop shop' for housekeeping logs, though. I'd also like to see it incorporate auditing housekeeping.