Greetings!
Your theory is absolutely correct. It is against vSAN design considerations to mix different type of disks in the same host and across different hosts.
VMware vSAN is based on RAIN architecture. If you want you can mix the size of disks within a vSAN Cluster (Please read - Using differently sized disks in a VSAN environment) but you should not mix the type of disks. The reason for this obviously performance issues. In vSAN, the objects are placed across multiple disks on multiple nodes. Consider a scenario, when your data is placed on two different type of disks (SAS and SATA) on two different nodes and you send a read request for that data. SAS disk will respond fast but the SATA disk will not be able to respond as fast as SAS disk and you will get latency while reading the data. Same applies to write operation as well. Hence, VMware strongly recommends using a uniform disk model across all hosts in the cluster.
Hope this is helpful.
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Cheers!
-Shivam