The database has a transitionally consistent memory cache. On a 5 node system, you have ~150GB of effective cache. And you can always add more nodes.

When you put a cache in front of the database, you sign up to manage your own cache coherence and consistency.

Are my users reading stale data? That write went into the cache, did it make it to the database? I use interface X to speak to the cache but interface Y to speak to the database. Some requests are ok to cache, some must go direct...

Or I can just get more nodes and not have to worry about it.