What questions should one ask when researching HCI solutions?

Availability, support, cost, compatibility and scalability, cloud readiness.

Can we do a proof-of-concept?

Does the solution support my critical/legacy application?

Does the solution support my current backup solution?

How does migration work, what downtime is to be expected (e.g based on hours/terabyte)?

Which other aspects are possible with this solution?

How responsive is vendor support?

It does not matter if the solution works natively with the hypervisor or through virtual machines which do the magic.
Native hypervisor integration will be likely a vendor-lock-in.
The main matter is "how well does it work"? See this through with my first question.

1. Support

2. Migration or Conversion process from existing solution

3. Cost 

4. Hardware compatibility 

5. Integration with all critical and non-critical solutions

6. Cloud readiness

> Integration with the existing running apps and solutions.

> Support Parameters.

> Ease of Scaling up and out the solution.

> Cost of the overall solution.

> Technical architecture of the solution.

> Integration with Cloud services/Solution should be cloud adaptive.

> Solution should be truly ready for the complete SDDC platform.

There are several solutions that claim to be HCI in the market, however the best factor is the native integration with the hypervisor without the need to have additional virtual machines that "perform HCI", so far in several cost-efficient scenarios that I have performed and in turn With different hardware manufacturers I can personally say that the best option is VMware vSAN. Its main strength is the correct management of hardware resources.

Collaboration

For me the most important component in an HCI Solution is the Software-defined Storage, so you always need to give great care when comparing SDS offerings from different HCI vendors.
You check the below points:
- Data Locality
- SDS Offerings (block storage, file storage, object storage)