Hello,
i think you have ask two different questions.
1. Its not a problem to run multiple VCSA on the same Hosts/Cluster or Infrastructure because it doesnt mean that a VCSA need to manage the underlying Hosts. Think about a dedicated Management Cluster with dozen of Multitenant Service VMs.
2. About the redundancy thats what vCenter HA is used for. It creates a 3 VCSA Active/Passive Application Cluster. Even in some larger SMB setup i dont see this often.
The VCSA LinkMode solves a different problem. Having a single pane of glass when running multiple vCSA on different location or purposes.
Regards,
Joerg