Both dramas feature emperor Qianlong's imperial harem so they share the same characters, but in Ruyi the main character is the Step Empress Lady Ulanara and in Story Of Yanxi Palace it's Empress Xiaoyichun, mother of emperor Jiaqing. Some characters are evil in one and good in the other.

Both are harem dramas and they share many similarities in the kind of schemings and tatics characters use to survive and also to get what they want. Empress Of China is set in the Tang Dynasty and Empresses In The Palace is set in the Qing Dynasty.

Both dramas adress the hardships and struggles faced by women in the jurchen/manchu culture. Rule The World is a time travel drama that covers the process of the foundation of the Qing Dynasty and Ruyi is a harem drama set in the reign of Qianlong, the sixth emperor of Qing.

Both dramas are set in the Qing Dynasty and adress the hardships and struggles of the palace life faced by women in that time period. Scarlet Heart is a time travel drama set in the reign of Kang Xi and we see Yongzheng rise to the throne. Ruyi is a harem drama set in the reign of Qianlong, son of emperor Yongzheng.

Both are harem dramas. Empress Of China is set in the Tang Dynasty, in the reign of Tai Zong Li Shi Min. Ruyi's Royal Love In The Palace is set in Qing Dynasty and the emperor is Qianlong, the sixth emperor of Qing, son of emperor Yongzheng. Main female leads must survive the cruel harem life.

Fun fact: in both dramas the main character's name is Ruyi.

Ruyi's Royal Love in the Palace is the sequel to Zhen Huan. But both are harem dramas and they can be watched without having to watch the other.
Zhen Huan covers the harem of emperor Yongzheng and in Ruyi, Yongzheng's son, Qianlong, rises to the throne and the story focuses on Ruyi, a consort of his harem.

Turbulent times. The modern and the old coexisting and clashing. Old ways and old customs are portrayed in a way that make them look strange and exotic.

Both dramas are very different from anything you expect from regular sageuks. They are visually very different and the characters are portrayed in a unique way to sageuks.

In both, a lookalike impersonates the "original". In School 2015, it's a twin sister, in Are You Human Too, it's a robot.

I honestly don't know why this recommendation hasn't been done after all this time these two dramas have been realeased. Both stories involve a deadly virus that has a 100% fatality rate. The End Of The World is focused more on the medical/science side and The Virus falls more in the crime (and of course medical) genre. Watch both and pick your favorite.

Beginning is very similar. Also both involve ahjummas uncovering mysteries and secrets involving their families. Mistress is a much darker show btw.

Having watched Woman Of Dignity it was impossible not to be reminded of it while watching Secret Mother:
The beginning is VERY similar. Also, both stories involve rich ahjummas, marriage, cheating, parenting, education etc.