Except when you're abroad on roaming cellular charges, and your three laptops decided that since your iPhone's personal hotspot is WiFi, it's time to download today's ChromeOS image version, because the one you had downloaded the night before was not good enough anymore.
Source: Chromebooks are awesome, and even with excesses like this, they're still the cheapest to operate by far.
Chrome OS tries to honor various DHCP server flags that state that the connection is metered. Unfortunately iOS doesn't seem to provide any such indication.
A comment in https://bugs.chromium.org/p/chromium/issues/detail?id=323010 claims that the BSSID is used for a "suspected" state, but that may not be enough to actually stop it from downloading updates, but I'm far from an expert in that domain.
In short, identifying tethering states with iOS seems to be hard.
You can determine whether the network is a Personal Hotspot heuristically. It is nice of them to have implemented private DHCP flags in Android, but if you routinely pull hundreds of megabytes without user interaction...
Source: Chromebooks are awesome, and even with excesses like this, they're still the cheapest to operate by far.