What could happen if cache_form is stored in Memcache?
Memcache will drop cache content to make room for new cache objects.

That may be a problem if using a single memcache server instance for several cache bins.

cache_form needs to be more persistent to avoid losing form build validation data.

You could have a dedicated memcache server instance/port for cache_form and that would work ok, provided the service isn't restarted. However, in practice, servers need occasional reboot which could cause loss of user form data.

Usually cache_form is stored in the database as it provides the semi-persistent storage required there.

If cache_form isn't working properly, your users will experience "This form has been outdated" and other form validation errors.