Both shows were produced by the same production company i.e. Daylight Entertainment, and therefore have very authentic settings and high quality production values.

Both shows take a bit of time to warm up as they introduce the main characters, the circumstance & situations surrounding them and the characters' unique personalities. But the storytelling in both is masterfully done so if one is patient, the pay-offs will be worth it.

Both shows have layered, complex and nuanced characters who have to face difficult life decisions and consequences for their actions.

Both shows explore themes of the tragic consequences of human nature i.e. corruption, injustice, loss and war. But both shows are more than just those themes, there are also heartwarming themes of friendship, familial love, loyalty and brotherhood.

Both shows are masterpieces with depth and substance.

They are both classic Jet Li films.

Both showcase Jet Li's immense martial arts prowess.

Both are set during a historical time in China with conflicts and underground resistance movements creating tension and issues.

'Fong Sai Yuk' has a more comedic vibe though, and so is less serious in tone from 'Once Upon A Time In China'.
Both have sequels made due to their popularity too.