I assume that you already know what Lockdown Mode is. With Lockdown Mode enabled ESXi server, you will need to use vCenter to Administrator ESXi Server therefore you can enforce centrally managed permission control via vCenter Server, rather than configuring and managing permissions at each ESXi Server.
When Lockdown Mode is configured on ESXi server, vCenter Server has to be highly available as Loss of vCenter Server makes your ESXi Server unmanageable. Luckily you have a workaround. if your vCneter Server goes down, and need to access ESXi Server, then you can login to DCUI (Direct Console User Interface) of ESXi Server to disable Lockdown Mode.