I imagine what they mean by being careful with the meters when using fast travel in hardcoe really just affects players if the meter is nearly out. I'm not walking everywhere all the time (I do wander alot but that's different than when I'm trying to complete quests)
I guess it's something liekt his. Still, it means that we shouldn't rely on fast-travel too much as it has the potential to mess up things. In short, we can't "abuse" fast-travel as much as we could in FO3 if we go for hardcoe mode. Well, I mostly only used fast-travel to avoid the metros or for a fast way to the nearest merchant when I was all stocked up on things I wanted to sell, so I guess I still won't use it all that much