The SFAPI and OData API are exposed for any web service integration, including Dell Boomi AtomSphere. The OData API can handle delta changes.
HCI is an evolving platform and offers similar functionality to Boomi in terms of building integrations with a drag-and-drop interface, etc. There is a SuccessFactors connector provided for HCI and the Talent Hybrid integration content is available on HCI as well as PI. The Employee Central integration content currently available on Boomi should be available on HCI hopefully by the end of the year. The Talent Hybrid high-level roadmap can be found here:
SAP HCM and SuccessFactors Integration Packages: an overview
Boomi is included in the Employee Central subscription, but is very expensive to license for other use cases (i.e. Talent Hybrid scenarios). HCI costs 7.5% of the SuccessFactors subscription. PI is free. HCI may be included in the Employee Central subscription at a later date, but this is not definitely going to happen.