In my case its mostly because I tend to run VMs on various older semi-retired machines with limited or slow local storage that I only turn on when I need them, and VMware's VMFS is cluster-aware, so it really doesn't matter which hypervisor is the one I end up spinning it up on.

I haven't dealt with Unraid specifically but there are a lot of caching and network parameters that can wildly affect performance -- VMware for example wants to do synchronous writes on network storage for obvious reasons, and having a safe write-cache and large transfers with enough in-flight commands can make a night and day difference.

If you're primarily just using NFS/SMB as file shares then getting iSCSI working probably isn't going to be a good use of your time versus figuring out why the existing setup behaves that way -- Samba and SMB performance tuning can be a frustrating experience but iSCSI is far more opaque and inscrutable on Windows particularly.