Ash can be a pain to farm because you have to find Grineer Manics. Specifically Grineer Manics, too; Drekar Manics (which are much easier to find) don't drop the pieces. But he's fun with the Fatal Teleport syndicate mod and a dagger with Covert Lethality.
Saryn is a pain to get because Kela De Thaym is one of the most irritating fights in the game, plus you have to keep doing arenas to get justice points before you can even face her. And then Chroma also requires Saryn systems to build, which is the least common warframe piece off her.
It's also a pain to get groups together for Atlas and Limbo just because everyone hates archwing missions, and people that already have those frames tend to have PTSD from the acquisition. I just bought both of them one day when DE had a massive sale.