Linux shared memory v mysql memory tables

Question

BACKGROUND

I'm currently working on quite a large system (for me) consisting of 30 or so applications, approximately 1200 web pages and 6 different databases. The system is one of 7 or 8 slightly similar systems in use and this one in particular runs on Debian wheezy and is completely isolated from the outside world.

Now, I have no documentation for these systems, indeed many have been ported from software written in the 80's in COBOL for example so debugging and maintenance it's pretty much a trial and error, such that I've since created virtual machines of several of the boxes, to avoid breaking production systems.

The applications mostly run in the background and respond to inter-process messages and do their thing. They also share 1000's of variables in shared memory with each application accessing and modifying if required, same with the web pages. I'm unsure which mechanism is used for the shared memory, having read about System V v POSIX here

I also note that certain applications write to database tables in memory and this leads me on to my questions below;

QUESTION

What are the advantages and disadvantages of using either shared memory or memory database tables to facilitate data sharing between applications and web pages (on the same machine)?