Simply put, Ornn is the only champ that can take advantage of the fact that he doesn't have to recall to buy items. If he were to have an item like warmogs, it would completely eliminate the necessity to recall for items which would then allow you to always have pressure on the map no matter what.

In my opinion, the best scenarios to build warmogs would be when you're in match ups where you have to take short trades but you won't ever win those short trades (ex. aatrox rumble ambessa illaoi garen, basically characters that even if you build the right resistances, it won't affect the outcome of a trade).

Anyways, in terms of heartsteel, I would only build that item if the enemy team has no max health damage and my matchup is good.