Zammad currently doesn’t seem to support more than one ElasticSearch node (see (How) can one configure multiple ElasticSearch nodes (`es_url`) in Zammad? - #3 by fthommen). This is a pity, as ElasticSearch is often deployed as an ElasticSearch cluster.
Not being able to configure multiple nodes of this cluster in Zammad IMHO defeats one of its primary purposes (see e.g. Design for resilience | Elastic Docs). Having to build a separate loadbalancer or an other failover mechanism in front of ElasticSearch also results in additional administration and maintenance overhead for the system administration.
I therefore suggest, that Zammad implements the possibility to configure multiple ElasticSearch nodes and not only a single one.