Both are LGBT dramas with heavy reference to the Red Thread myth of fate (I honestly wish more dramas had plots with this)
Apart from the obvious difference that YMMD is a kdrama and UWMA is a Thai drama, YMMD is kinda lighthearted and short, while UWMA starts with a dark undertone (don't worry, it gets fluffy later) and is longer.
Besides that, UWMA is a must watch for BL fans - I can't help but gush over it!