I think what RONIS want to say.

Let's say that primary storage gone off-line suddenly (power outage, UPS failure...).
Content of RAID write cache is safe since there is BBU installed.
But, the content of RAID write cache still isn't written to HDDs.
Content of HDDs (on primary storage) is synchro replicated to HDDs on secondary storage.
But, in that moment, HDD on primary storage has data which still isn't updated because updated data still resides on RAID write cache.
And, secondary storage will not have updated data when it take over and become active storage.

This is serious question, and need an answer