A few weeks ago I wrote about several critical issues with vSphere 5.5 and 6.x versions. The 5.5 version was affected by a serious bug that causes VMs to crash during snapshot removal processes. That made backups a real challenge because you had to choose between having backups or keeping your VMs running. Not a decision you want to make.
vSphere 6.x was hitten by a random loose of network connectivity for the service console port so a stable use was nearly impossible.
Fortunately for both problems VMware released bugfixes. For vSphere 5.5 Update 3a is now available, for vSphere 6.x Update 1a fixes the problem. You can get access to these bugfixes by simply using the Update Manager or if you prefer to get some more information you can follow these links to Update 3a and Update 1a.