The primary challenge was just reasoning with the template and using Helm at scale... ie; what exactly did we deploy on those hundreds of varying clusters?

Other issues included; tiller would sometimes become unstable... version mismatch issues between helm local and roller... lack of a clear, outage free canary deployment... we even found cases where helm would not cleanup after itself during a deployment and retain previous config settings within k8s.

For me, Helm caused more problems than it solved. Pulling in packages always seems good, but as soon as you want customization, you're back to merging in the (relatively) straight-forward yaml files from the chart. Also, instead of Helm's templates (which get crazy complex and unintuitive), a simple tool like Kustomize[0] is very straight-forward and allows per-environment configuration. Finally, tiller pods do present (yet another) security risk for Kubernetes.

[0] https://github.com/kubernetes-sigs/kustomize