In one sense this is true, but if you rely heavily on AD integration and passive authentication a FMC outage can becomes a serious problem.  Your AD agents or ISE is relaying all your user to IP mapping through the FMC back to the individual firewall...

Here it is months later and we just upgraded FMC and some SFR and FTD to 6.6.1 and are seeing the same issue for a handful of sites currently.  cn.ca is an example.   Did you open a tac and get this resolved?  I have TAC opened but not much action so...

How did you load the IOS?   I use the archive command with tar files and the stack version seems upgrade or downgrade with the ios.