A while ago I wrote:
I haven’t seen any hard data, but intuition suggests that apart from hardware failures a standalone firewall might be more stable than a state-sharing firewall cluster.
I don't have statistics to prove it, but as a hoster, I can confirm that I've seen high availability appliances fail more often than non-clustered ones. And it's not limited to firewalls that crash together due to a bug in session sharing, I have noticed it for almost anything that does HA: DRBD instances, Pacemaker, shared filesystems...
It fails, fails and fails. There are bugs, but also configuration issues/bad sync, network flapping that breaks replication and puts all instances in standby/secondary mode...
Same for VMWare clusters that cannot handle partial storage loss or performance issues without losing it completely, disconnecting from vCenter because their daemon indefinitely blocks on refreshing storage status.
In the end, a non-redundant VM on a non-clustered host seems to be the service that lasts the longest.
The good compromise for most people that do not require very low recovery time seems to be non-redundant VM(s) running on a cluster to cover host failure (hardware or software), and maybe "passive" VM(s) on another cluster that is triggered manually.
And if customers need very high availability, they should/must design their app to take it into account and host it on multiple locations/independent providers.
Not that many people would want to listen to anything along these lines. Believing in Santa Claus and magical High Availability solutions is more comforting.