Benefits of running 2 Wi-Fi networks from the same router? What are the downsides? (I don't know if there is a better community for this question)

Energy consumption is essentially the same, as it's using the same radios.

For what it's worth, I have several SSIDs, each on a separate VLAN:

• my main one
• Guest. Has internet access but is otherwise isolated - Guest devices can't communicate with other guest devices or with any other VLANs.
• IoT Internet: IoT and home automation devices that need internet access. Things like Ecobee thermostat, Google speakers, etc
• IoT No Internet: Home automation stuff that does not need internet access. Security cameras, Zigbee PoE dongle (SLZB-06), garage door opener, ESPHome devices, etc

(to remotely access home automation stuff, I use Home Assistant via a Tailscale VPN)

Most of these have both 2.4Ghz and 5Ghz enabled, with band steering enabled to (hopefully) convince devices to use 5Ghz when possible.

This is on a TP-Link Omada setup with 2 x EAP670 ceiling-mounted access points. You can create up to 16 SSIDs I think.

Are we talking main + guest network, or 2,4GHz + 5GHz, or something else?

What benefit do you hope to get?

Whether it has benefits is up to you, but from a technical perspective they're as expensive as VLANs, so basically free. It's the same receive and transmit radio, the only difference is that it broadcasts and responds to two network names at the same time. The maximum power consumption is the same: the max the radio will pull when at full load. The minimum power consumption has to be ever so slightly more since it needs to broadcast two network IDs, but those are measured in bytes and sent a couple times a second, it's negligible compared to the cost of just running the radio.

I remember reading a few self hosters describing having a separate WiFi for IoT devices, on a dedicated router (opensense) so they can prevent these devices "calling home". They are maybe other advantages like having different WiFi channels for these things

Separate subsets, segregated traffic. Easy to avoid crosstalk by setting channels further apart or using 2.4ghz and 5ghz

At home I have one SSID as a main wifi, and the other is guest wifi and IoT or other random devices.

Main downside is getting it setup and maintenance.

I'm not sure that I understand the "more noise and energy consumption" part, since we're still talking about the same router with the same connected devices.

But I do have multiple SSIDs on my router. One is explicitly for IoT devices, and they don't have network access, so they are isolated from my computers, NAS, etc.

I am worried that there is not really a benefit of doing that, just more noise and energy consumption.

If there wasn't a benefit, why would people (and pretty much every business) do it?

I think most wifi routers segregate the two networks, so they can't see devices on eachothers network.

Someone will surely correct me if I'm wrong in this.