With HCI typically you can set it up & scale/downsize infrastructure easier compare to CI.
I would make an analogy with homes. There are handmade costly and stylish houses, and on your land, as with Lego you can do whatever you want to, but it means, you need to do that whatever you been thinking of.
And there are excellent and comfortable condos with many amenities in it, but not to go beyond that.
So with FlexPod, you can have FAS/AFF, SolidFier, E-Series, blades or rack servers, many OS, Hypervisors and applications and it is not limited to existing architectures & designs.
With HCI you can quickly start, scale/downsize, but it has only three types of computing nodes & 3 types of storage nodes, and it is only VMware.