Which is usually a correct assumption. But it's more than just metered, it's also assuming that my wifi connection is likely to be faster and therefore downloading a bunch of assets in the background will have less of an impact on my available bandwidth.

'usually' you mean in the US?

While I was in the UK I had unlimited data on my mobile but a 100GB limit/month on my landline connection

Metered landlines aren't uncommon in the US, especially in areas with poor telecom infrastructure. I think if your app is worried about amounts of data relevant for metered cellular (which can be in the 2GB range in some places, including the US), the metering on landlines is virtually infinite in comparison.

I know enough people who's mobile data is both faster and cheaper than their home broadband that I cannot agree to your statement that this is usually a correct assumption.