I think, vCenter Update Manager is the best tool for upgrading ESXi.
vSphere Auto Deploy is useful for new host deployment.
I agree, but in Auto deploy we just need to boot from golden ISO file that's it, which will save time, isn't it?
This depends on the requirement. Auto Deploy will help to boot/Install from network. The existing configuration is only saved when the image installed is upgrade.
You can choose to do a network image boot with host profile applied to configure the host.
Will this be quicker than an upgrade?
Well, I would say no since this also need a host reboot and host profile apply task would match similar to a host upgrade task as well.
One of the biggest problems with Auto Deploy is how complex your Host Profiles may need to be in order to match your environment. We are using this method in some areas (with stateless hosts) but in others we still use VUM.
Auto Deploy is pretty easy by itself and is a great way to get the host code/vib's upgraded. It's everything else you may have to do to get the host configured to the way it was that is the problem. We have issues with VSAN environments, such as won't add "Virtual SAN Traffic" to VMK, HA agent doesn't deploy properly, internal USB drives don't show up until a RESCAN of host, etc...
We have thousands of hosts with numerous vCenter servers and still rely on VUM.