Flux 2 supports these use-cases via its Remote Apply Feature, Path+Ref specific Sources, and Events/Metrics.

Remote Sync

Flux can ensure workloads are applied, health-checked, and garbage-collected in remote clusters.
See these docs for Cluster API considerations:

• Kustomization: spec.kubeConfig
• HelmRelease: spec.kubeConfig

These two demos do remote applies from a central management cluster:

1. ClusterAPI + Flux + GPG verification
   Repo: https://github.com/stealthybox/capi-flux-demo
   GitOpsDays Video: https://www.youtube.com/watch?v=7W27tAv7Tvs

2. Multi-Cluster CNI management /w DNS Mesh & Route Sharing
   Repo: https://github.com/stealthybox/multicluster-gitops
   KubeCon Video: https://youtu.be/as4NU-Z8dlE?t=1244

Configuring apps for environments/namespaces/clusters

We have this example regarding releasing apps to multiple environments and clusters

• Power of GitOps Part 3
  Repo: https://github.com/stealthybox/flux-app-demo
  Video: https://www.youtube.com/watch?v=N_K5g7o9JKg

Separate paths in a Source can be used to modify a kustomize base (or other manifest generator).
This can be done per-environment.

Environment specific tags can be used for the GitRepository ref alongside those paths to release the desired version of each environment separately.
This allows you to commit changes to all environments at the same time while using tags to rollout changes incrementally.

Viewing all sync info from a single place

When using a management cluster /w remote sync via the Kustomization or HelmRelease spec.kubeConfig, all sync state (object names/errors/commit-shas/etc) can be read within a single cluster.
If desired, all of these sync related resources can be in a single Namespace and be viewed simultaneously using a tool like Octant, Lens, k9s, the upcoming Flux UI, or even just kubectl.

This data can also be scraped from the controllers via Prometheus and displayed in a Grafana Dashboard.
If you prefer, to run a Flux install per-cluster without any central management cluster, these metrics could still be aggregated into a central reporting Prometheus/Grafana.

You can also hook up Alerts for Providers using notification-controller's API's to be notified of any issues via a chat channel or other paging service.