People are really forgetting how these two items are calculated, the returned damage from thornmail is after all calculations, it is 30% of what the wearer actually took, if an AD carry crits a tank for 500 damage they will take 30% of that, which is 150 damage, if the tank has 9 magic pen and 10% magic pen against a target with 30 MR they will have (0.9*30)-8.5=18.5, at 18.5MR they will receive 84.4% of that reflected damage, .844*150=126.6, now if they have even a single Bloodthirster, they will negate nearly all this damage, since 20% of 500 is 100, they would then only take approx. 26.6 damage.

This is in a best case scenario, excluding an Abyssal Scepter or other form of reduction, even then, an AD carry will often/may have a GA and/or MR/level runes which will drastically reduce this received damage, which ultimately make purchasing the item worthless when compared to the alternatives, the same can also been said for the Sunfire Cape, since they will likely be taking much less than 40 magic damage/sec, usually less than 30, which over a 10 second encounter is only 300 damage, taking into account life steal this is also mostly negated.

Also, if you take into account melee bruisers, you can throw it all out the window and stamp "completely worthless" on these two items for their unique effects since they will likely have over 100MR.