Both dramas are about married people who discover their soulmates while also being trapped in loveless or toxic marriages. Both dramas question the morality of adultery and what is considered "acceptable" when divorcing or separating have larger consequences for the leads and their families.
If you like petty, selfish male leads, an indecisive female lead, frustrating love triangles, head-scratching writing, and dramas that start off fresh and unique only to end in embarrassing disappointment, this may be the drama for you. Both have amazingly talented casts who do a stellar job portraying their characters despite how badly they're written.
These are both intense dramas that feature revenge - as with Secret/Secret Love, the male lead's entire motivation is shaped by a former/first love, and the on/off relationship with the female lead. The actors in both dramas have AMAZING chemistry together, but their parings have some wild moments that make you question their sanity or common sense. If you like dramas with over-the-top and toxic OTPs, this may be right up your alley. Both have some very triggering content (violence, abuse, assault, etc.), so I wouldn't recommend either drama for younger viewers or for those who may be affected by the things listed above.
They both feature revenge plot elements, with the male lead's entire motivation structured around his love/hate relationship with the female lead and the loss of his former love. They both also have heavy instances of cringeable moments (domestic violence, abuse, torture, sexual assault), and the love relationships that develop between the leads in both dramas so toxic and inexplicable that it borders on makjang territory. Both Le Jun Kai and Secret Love are well-written, well-produced shows with very strong actors who have excellent chemistry together, but neither drama is suitable for younger viewers or for those triggered by the things listed above.