Both romcoms have the same funny dynamics between a cold and arrogant boss and his warm assistant.
Only difference is the slight supernatural twist in Masters Sun.

Both have action, a mystery and two hotties going from bickering to jumping each others bones, with love of 18+ moments between them. The Sign has a bit of a supernatural twist, but they still feel similar.

Both have a seemingly age gap between a grown man and an high-school girl, but somehow both dramas manage to be safe and fluffy. Both dramas are funny and full of banter, with the girl giving as good as she gets.

While different in tone and approach, both dramas deal with age gap romance in a sweet and mature way. They address it respectfully, but they both see the lead couple as having personality traits that go beyond their biological age.

Both dramas owe their existence to the works of Arthur Conan Doyle, although with Miss Sherlock it's more obvious.
Both feature an investigation pair with one partner being a genius and the other coming along for the ride to keep the genius safe and grounded.
Both dramas leave you with a craving for a second season.

Both have slowish mutual respect and then romance between a cold well off official and a warm but smart woman who works with him.
Both have past and present royal plots and betrayals that need to be resolved.

Both are BLs focused on young men who want to be musicians, so both have great OSTs.
Both dramas are short and sweet.

In both dramas the smitten ML makes a move after the FL gets dumped.
Both dramas are fluffy rom-coms.

Both dramas feature a FL that is jaded because of her paranormal ability.
Both dramas have a mix of humour, romance and cynicism.

Although the setting is different, both are BLs and in both one of the MLs is a sweet psycho.
In both dramas thet wto MLs complement each other in temperament and strengths.

Both are romantic comedies with a couple from two different worlds.
In both dramas the ML is stronger than the FL and has beastly traits.

For the viewers who get a kick out of watching badly done dramas, they are both such train wrecks that it's amusing to watch.