Disadvantages of using bit fields in memory

I work on a system with limited x64 memory, and I need to store a lot of data in memory. What are the disadvantages of using such a data structure.

struct entity
{
  unsigned int hash : 26;
  unsigned int timestamp : 14; 
} __attribute__ ((__packed__));

I know that using bit fields is not recommended, but what are the worst flaws in using this inconsistent data structure.

The structure will be used in memory, and some performance degradation is expected.