Setup MongoDB replica servers with different power?

Is it a good practice to setup replicas with the same machine size (wrt. CPUs, RAM, Bandwidth, etc.) or can replicas be based on smaller machines? One usecase might be to have a small non-voting backup replica machine. Another is to save money on the replica "backend".

The only problem I see with voting replicas is that in case the primary (big machine) goes down the new primary would be weaker. Are there other problems with clusters using different machine sizes?