In recent discussions with Splunkers and customers, I keep hearing about how the plan is to launch investigations in Phantom. In the recent past, all I ever heard was about how ES was the location where investigations should be managed, due to the provided investigative workbench framework. I've got zero experience with Phantom, and generic statements about investigative features in both products, leaves me confused.
This is something I have been trying to figure out as well. The way I see it as I am learning both products is that with Splunk ES, investigations work well adding context and artifacts from searches within your Splunk environment. You can gain insight and context around an event from your log sources, but it doesn't work well with external intel like data enrichment from VirusTotal for example.
With Phantom, Splunk ES notable events seem to be your primary source of events in Phantom and the platform allows you to add context and intel from external sources. Phantom can run reputation of artifacts and display the results really well. It can also easily detonate files and URLs and provide findings, which can also have automated actions taken from the results of any of those actions. It doesn't seem like it can easily provide statistical data from your event logs though. Splunk has swimlanes and timecharts for example that might show trends and anomalies over time.
My hope is to see more integration between the two products. I would like to see Phantom integrated in to the Splunk ES investigation workbench or bring some of the Splunk dashboards over to Phantom more easily. I feel like both products have two separate pieces of the same puzzle and there are advantages and disadvantages to using either product as your primary alerting source.