Host upgrades with custom images offer extended driver support for vendor specific hardware or agents. You’ll get drivers that are not included in a standard VMware (Vanilla) image. Upgrading with customized images may lead into trouble while updating existing driver packages. There used to be a nasty bug with the lsiprovider package on Fujitsu ESXi 5.1 images. Another example was the “death by upgrade” bug (blog post in German) when upgrading a customized Fujitsu installation to ESXi 6.0. There are other examples from different vendors in the hall of shame.