I heartily disagree. Mummy's Mask (by far the best set sofar), introduces far more new ideas and concepts than Skulls & Shackles and Wrath of the Righteous combined.
The introduction of the trigger mechanic adds risks to examining card stacks. There are characters and cards which directly manipulate check results, adding or substracting numbers. In turn there are banes that need to be defeated by a margin to give benefits or hindrances, making these number manipulations valuable. The trader mechanic makes acquiring boons more interesting, as you have a chance to acquire boons of your choice after a scenario.
And all that before considering the setting, which as many uniquely themed near-eastern, egyptian flavoured cards that are not reproduced in the other sets.
In my opinion, the set that plays closest to RotR is Skulls & Shackles, as the major new rule are the boats and loot.