Never got into it that deep myself, I just know other people who never had issues. Prime-time streaming in full hd etc.
But I'm also pretty sure you can sue them, if they can't keep up the advertised speeds over longer time. Obv only when the infrastructure is actually available.
For unlimited data contracts you can usually pay different amounts for different speeds. They actively advertise with those maximum speeds and if you can never reach them, even tho they are available at your location, you can report them to a federal agency and take legally warranty claims.