I'm using page+object+db cache and preloading with memcached and can not confirm any ot these problems. Although the size and number of connections for memcached should be set properly.
If size is to small the 'Evicted' counter in the stats is nonzero and grows with time. You can display stats using the command memcached-tool 127.0.0.1:11211 on the console. See Memcache Telnet Interface for more details.
Also Memcached Internals for End Users gives a clue how memcached works internally.