Proxy load balancing docs first say:

When monitored in proxy group the VMware hosts will be randomly spread between proxies in the group and will cause each proxy to cache all VMware data causing additional load to Center.

then:

vCenter monitoring cannot be used with proxy load balancing

The latter statement contradicts the first one.

Additionally:

anything with host prototypes will cause trouble

This is not true, proxy load balancing works fine with prototyped hosts.

Also, please add that only agents 7.0+ work with proxy groups in active mode.