Update Manager is not involved at all. HP SUM is the easiest method, because it can be launched from the SPP DVD without any further requirements. The Insight Control plugin requires a vCenter server and an HP SIM installation.
Firmware updates are as disruptive as with other OS. They can be done online without entering maintenance mode, but some of them (e.g. server or Smartarray BIOS) will require a reboot to become effective (which is not forced).
Please note that NIC firmware updates are not available as online update components, and must still be applied by booting the ESXi host from the SPP DVD.