Is making kubernetes masters schedulable a really bad idea?
https://kubernetes.io/docs/tasks/administer-cluster/highly-available-master/


  Best practices for replicating masters for HA clusters
  
  Try to place master replicas in different zones. During a zone failure, all masters placed inside the zone will fail. To survive zone
  failure, also place nodes in multiple zones (see multiple-zones for
  details).
  
  Do not use a cluster with two master replicas. Consensus on a two-replica cluster requires both replicas running when changing
  persistent state. As a result, both replicas are needed and a failure
  of any replica turns cluster into majority failure state. A
  two-replica cluster is thus inferior, in terms of HA, to a single
  replica cluster.
  
  When you add a master replica, cluster state (etcd) is copied to a new instance. If the cluster is large, it may take a long time to
  duplicate its state. This operation may be sped up by migrating etcd
  data directory, as described here (we are considering adding support
  for etcd data dir migration in future).


I would like to emphasize the following quote


  A
  two-replica cluster is thus inferior, in terms of HA, to a single
  replica cluster.


If less then 3 nodes will be used then it is basically more safe to run 1 master, but if one goes down there will be an issue as well. So it is better to run at least three masters.