What are the key performance advantages of in-memory databases vs disk based NoSQL databases?

Reading Designing Data Intensive Applications book, I encountered this statement:

Counterintuitively, the performance advantage of in-memory databases is not due to the fact that they don’t need to read from disk. Even a disk-based storage engine may never need to read from disk if you have enough memory, because the operating system caches recently used disk blocks in memory anyway. Rather, they can be faster because they can avoid the overheads of encoding in-memory data structures in a form that can be written to disk. OLTP Through the Looking Glass, and What We Found There

So my question are:

  1. Given that a disk based NoSQL(Mongo DB) is given the same amount of RAM as an in-memory database(Redis), will they perform about the same?
  2. What if a disk based database used write back caching strategy with async writes to persistent storage with both having same amount of RAM. Will that make it's performance similar to an in-memory database?
  3. Even if serialization has such a high penalty (as mentioned in the quote above) would disk based NoSQL match the performance of in-memory databases for a read heavy system with same amount of cache given to both databases.

I'm very new to the NoSQL world so please do direct me in to the right direction if I missed something.
PS: I've read Difference between In memory databases and disk memory database but it doesn't address my specific questions.