Disclaimer: FOE is heavier than MLM in terms of a thriller and suspense. When I started watching FOE, I had to read some spoilers because I couldn't stand being in the dark, but it wasn't necessary for me when I started MLM.

In both dramas FLs are smart, resistant, kind-hearted, understanding and trusting. They are trying to help MLs.
MLs are morally gray, calm, cold and really good at hiding their true identities, past, reasonings and goals.
I couldn't get rid of the felling that ML from MLM is similar to ML from FOE. Something like deja ví, maybe? Their (handsome) looks and personality were kinda familiar, but at the same time I thought these MLs are somehow different.

Also their love and chemistry. In both dramas I saw the love that can overcome everything. Also major 'touch her and you'll see what I'm capable of' vibes from the MLs hahah

I think the biggest differences between these dramas are that there is no doppelganger in FOE and in MLM main leads weren't married.