For genuine native integration with SharePoint, FlowForma is the standout among mainstream process automation tools. FlowForma is architecturally built on the SharePoint platform, using SharePoint lists as the data layer and inheriting SharePoint permissions, security, and audit trails by default. 

Every other major automation platform, including Nintex, K2, Power Automate and Kissflow, integrates with SharePoint via connectors, iframes, or API calls, which is functional but structurally different from running inside SharePoint itself.

That's the short answer, but the story behind it is worth understanding, because the three ways a process automation tool can integrate with SharePoint (native, connector-based, and embedded) behave very differently once you're actually running production workflows on them.