Should config via cli rather than gui?

I agree with Aswin on management, configuration, and troubleshooting.
Regarding HA, I would say you must configure changes on the active chassis to see the impact of the change. If you make a change on the standby chassis, you cannot confirm whether it works fine or not.

Also, we have two options for configuration sync: Automatic and Manual. If you use the Manual option, it provides a safer way to roll back the configuration. Make the change on the active chassis first; if everything works well, you can sync the configuration. Otherwise, you can sync the configuration from the standby chassis to the active one.