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xkcd #3109: Dehumidifier
(imgs.xkcd.com)
Title text:
It's important for devices to have internet connectivity so the manufacturer can patch remote exploits.
Transcript:
[A store salesman, Hairy, is showing Cueball a dehumidifier, with a "SALE" label on it. Several other unidentified devices, possibly other dehumidifier models, are shown in the store as well.]
Salesman: This dehumidifier model features built-in WiFi for remote updates.
Cueball: Great! That will be really useful if they discover a new kind of water.
Source: https://xkcd.com/3109/
So, first off, smart devices shouldn't need to connect to the internet, only the local network. I have everything connected to Home Assistant, and then for access outside the house I have HA connected to the internet, meaning I only have one point I need to secure.
On your second point, I think the poster above was talking about having both an in-built as well as wifi-accessible external sensor. It makes it possible to have a more powerful dehumidifier in one space, running to a lower humidity than needed based off what's going on in other rooms. Then have that air circulated by other fans, etc.