Don't require ARM deployment to use Managed Identity
Per https://docs.microsoft.com/en-us/azure/service-fabric/how-to-deploy-service-fabric-application-user-assigned-managed-identity , managed identity can only be used when Service Fabric applications are deployed and upgraded via ARM templates. While it's great to have ARM template support, I've found it to be buggy, slow down deploys and upgrades, and in general it creates yet another layer in the tooling that is opaque and difficult to troubleshoot.
If using Azure, it should be possible to use Managed Identity in SF apps without requiring that all SF deploy operations go through ARM.
John Crim commented
1 data point to support this: after getting ARM app deployment working, each SF app deployment through ARM (for my app) takes 5 minutes. In contrast, a deployment or upgrade using the servicefabric client (C# code) takes 37 seconds. Fast build + deploy loops are critical for modern development, and a 4.5 minute addition to the critical path is significant.