Could also be an issue if a vendor-specific ISO was used to initially install ESXi. I ran into a similar problem with Dell; I had to get the Dell ISO from their downloads for the specific server model/service tag.
For 6.0 this is fine (you'll use this to get ASNC drivers). You can also add the dell VIB depot to get them this way also.
For 6.7 Dell has (thankfully) stopped shipping ASYNC drivers and moved 100% to inbox. It's all for 3rd party management VIBs. Honestly I've seen 3rd party VIBs be a culprit for updates before. I'd make sure you are updating the BIOS/FIrmware also when updating ESXi (People forget this sometimes).