I think the answer largely depends on the devices you use. Many devices require an internet connection to integrate with HA. Fortunately, each HA integration should list whether or not they can work locally.
Here are some device suggestions:
Bluetooth
Zigbee/Zwave/Zwhatever (local radio control). I don’t use this, but a lot of HA gripes I notice tend to center around support for these devices.
The HomeKit spec requires local only control at least as an option. HA can act as a HomeKit hub through the HomeKit Devices integration, so you don’t need any Apple hardware to use HomeKit devices.
Shelly makes great wifi and Bluetooth devices that work locally.
If you get wifi devices, put them on a separate network and/or have firewall rules that deny them internet access.
Use a VPN like Tailscale to access your HA from the internet, rather than exposing your HA instance to the internet through port forwarding.
Just as an example, I have Ecobee thermostats that are HomeKit compatible. Ecobee provides a cloud service, but I don’t use it at all, and my thermostats are denied internet access at the network level. They still work great through the HomeKit Devices integration.
I think the answer largely depends on the devices you use. Many devices require an internet connection to integrate with HA. Fortunately, each HA integration should list whether or not they can work locally.
Here are some device suggestions:
Just as an example, I have Ecobee thermostats that are HomeKit compatible. Ecobee provides a cloud service, but I don’t use it at all, and my thermostats are denied internet access at the network level. They still work great through the HomeKit Devices integration.
Good luck!