I...I don't think you understand how these work? Especially how they're installed in enterprises.
As a general rule, enterprises used to buy hardware dedicated to a specific project or workload. Each DB had it's own SAN, it's own servers, etc. But then we found that this was spectacularly inefficient and led to massive underutilisation of resources. Private clouds - or at least virtualisation setups that were closeish - began to become the order of the day.
Resources began to be purchased and pooled based on cumulative predicted need, not based on the individual project or workload. Now the question has become "how best to maintain these sorts of environments."
Something like a Nutanix or VSAN cluster rarely goes beyond 16 nodes, sometimes to 32. You get multiple clusters in a virtual datacenter. You are highly unlikely to have nodes in the cluster that are different speeds/capabilities because clusters tend to live and die as a group. We've seen that even in non-VSAN clusters thus far. Clusters are born, they live and they die as one.
But in the rare instance where clustered are mixed - I run a mixed cluster myself - sysadmins can simply tell workloads to keep the copies on "like" nodes. If you have PCI-E storage on nodes A-D and only SAS storage on nodes E-H, then you can "segregate" the cluster into two.
In theory, you could end up with a workload split along the storage plane, but only if you'd lost enough of one type of node that rebuilding would cause it to put the second copy on the other class of node. As soon as you've repaired the server sin question, policies would take over and make sure your workloads go where they are supposed to.
If your assertion is somehow that server SANs are unable to support SQL or OLTP workloads, well...you're just wrong. You're wronger than wrong.
Believe it or not, server SANs have been around long enough to evolve to handle the concept of diversity in workloads...and to handle workloads that are as demanding as anything you could throw at a traditional SAN. Indeed, I'd challenge a traditional SAN to keep up with the all-flash server SANs. The MCS setups in particular are utterly spectacular.