Tanks getting wave-clear and extra damage WHILE acquiring efficient tank stats is the problem.
When Nautilus and Malphite are able to split push faster than damage-oriented Top laners there is a problem.
As you stated... Gauntlet isn't the best defensive item, as it gives a good chunk of armor but zero health. The active damage in conjunction with the CDR give tank champions TOO MUCH utility. It solves their mana issues, it gives a massive 20% CDR and it gives you a slow and extra damage.... so much utility for a relatively cost-efficient item.
Sunfire Cape however is a very strong item. You have to remember that the damage is PER SECOND. so if cape is doing 20 dps, and you are in a teamfight, if you are around 2 people for 10 seconds you just did 400 damage... just for being alive.
Too much free damage at such an inexpensive price-tag. It takes damage carries so long to be able to respond with meaningful damage.
Iceborn Gauntlet and Sunfire Cape cost 2700 gold
Lord Dominik's Regards costs 2700 but sucks if you're not attacking a tank
Black Cleaver 3500
and thats about it for shred options... dont get me started on why the executioner's calling version is so pathetic.
Either bring back the old Last Whisper, or nerf the damage output of tank items.