API gets thrown around pretty loosely, but what Hubspot has isn't really an application programming interface. It is a state transfer service, as echoed in the documentation. While am I sure their service is not free of flaws (the versioning as described comes off especially strange), the biggest issue here may be taking "API" too literally. What you are looking for is, indeed, an API, but that is not what they have…
> HubSpot’s developer platform is a core part of our mission to empower organizations to grow better. Our APIs are designed to enable teams of any shape or size to build robust integrations that help them customize and get the most value out of HubSpot. All HubSpot APIs are built using REST conventions and designed to have a predictable URL structure. They use many standard HTTP features, including methods (POST, GET, PUT, DELETE) and error response codes. All HubSpot API calls are made under https://api.hubapi.com and all responses return standard JSON.
I'm getting a bit more estate transfer implications in their old docs but barely
> The API is built to allow you to create a functional application or integration quickly and easily. We know from experience - these are the APIs that power the HubSpot application. The ecosystem of developers creating integrations on top of the APIs is strong and diverse, ranging from webinar providers to CRMs to social media.
Not really sure where they mention it as a state transfer service. Not to mention even if it was, their lack of support for over 50% of their workflow/sequence/queue automations products make state transfer impossible anyways even for internal teams.