Recently I saw the following tweet from a well-known VMware blogger
And had flashbacks to some nightmares I had back while working at Cisco. At Cisco we commonly had to build custom ISOs to handle a variety of custom drivers (Nexus 1000v VEM, VIC drivers for Eth/FC, VM-FEX driver, etc). If you didn't do this right you were SOL, and if you did choose "ignore compatibility warnings" to upgrade anyway, then you were likely going to have to reinstall. Looking at the official guide to creating a custom ISO, you've got a lot of steps to get right and won't know if you were successful until you try to install.
