What is exactly a uniform server in a galera cluster setup?
The word "uniform" is being used to describe the fact that each server should have similar or identical resources, in terms of cpu, memory, disk (seek time, I/O bandwidth), etc.

Since a single node can slow down the entire cluster, the slowest node determines the overall capacity of the cluster... so using servers whose performance capabilities are not "uniform" (closely comparable) will mean that resources on the more powerful machines will go underutilized, while the less powerful machines slow down the cluster.