It looks like only Elasticsearch 2.x is supported. I've got this error with Elasticsearch 5.x:
[PipelineException: Failed sniffing cluster state.]
Elasticsearch.Net.RequestPipeline.Sniff() in C:\code\elasticsearch-net\src\Elasticsearch.Net\Transport\Pipeline\RequestPipeline.cs:340
Elasticsearch.Net.RequestPipeline.FirstPoolUsage(SemaphoreSlim semaphore) in C:\code\elasticsearch-net\src\Elasticsearch.Net\Transport\Pipeline\RequestPipeline.cs:134
Elasticsearch.Net.Transport`1.Request(HttpMethod method, String path, PostData`1 data, IRequestParameters requestParameters) in C:\code\elasticsearch-net\src\Elasticsearch.Net\Transport\Transport.cs:53
Elasticsearch.Net.ElasticLowLevelClient.IndicesExists(String index, Func`2 requestParameters) in C:\code\elasticsearch-net\src\Elasticsearch.Net\ElasticLowLevelClient.Generated.cs:2514
Nest.LowLevelDispatch.IndicesExistsDispatch(IRequest`1 p) in C:\code\elasticsearch-net\src\Nest\_Generated\_LowLevelDispatch.generated.cs:1242
Nest.ElasticClient.Nest.IHighLevelToLowLevelDispatcher.Dispatch(TRequest request, Func`3 responseGenerator, Func`3 dispatch) in C:\code\elasticsearch-net\src\Nest\ElasticClient.cs:56
LowLevelDesign.Diagnostics.LogStore.ElasticSearch.ElasticSearchClientConfiguration..cctor() in C:\a\1\s\LogStores\LowLevelDesign.Diagnostics.LogStore.ElasticSearch\ElasticSearchClientConfiguration.cs:77
, which is confusing, since given ElasticSearch versioning scheme it's easy to assume that 5.x is OK.