Why is Hash index not transaction safe.

Hi all,

I read the following about Hash indexes in Heroku's blog (
https://devcenter.heroku.com/articles/postgresql-indexes)

*Hash Indexes are only useful for equality comparisons, but you pretty much
never want to use them since they are not transaction safe, need to be
manually rebuilt after crashes, and are not replicated to followers, so the
advantage over using a B-Tree is rather small.*

Could anyone explain about why is it not transaction safe as compared to
B-Tree index.

Thanks!
--
Regards,
Ang Wei Shan