They have the same vibe.
Both Dramas are heartwarming.
Both Dramas also have Sports in it.
And the Characters never give up in both drama.

They really have the same vibes.
Female Lead Characters are both Cute and Sassy. They also have the same Genre Friendship,Youth,Romance and Drama.

Reply 1988 is set in 1988 while Twenty Five Twenty One is set in 1998.

They're both similar on Friendship
Both drama executed their close friends first.
Male Lead's Character are also similar

Both ML is suspected in a serious case. Lot of evidence that can prove that they're the killer but Who knows?!
They have the same vibe because of the genre Crime Thriller.

They're pretty similar because of the Death Game... Even not literally the same they have lot of similarities. Death Bell is highly recommended for those people who like Squid Game. Definitely not for the faint hearted

-Similar Revenge Plot. ML comeback etc.
-Rivalry
-Flashback Romance
-Period Dramas
-Strong Male Lead
-No peace in their town(Kom Faek) while the King is worst in Joseon(Queen for Seven Days)

There's so many difference between this Drama but it has the same Time Travel...
Both Leads go back to past

They're kinda similar with the genre and tags

FL on The Hymn of Death is a professional suprano singer while in Luk Krung, ML is a great balladeer.

Both is Forbidden Love also and the Main and other Supporting Characters are very similar in many ways.

Both Drama has Strong Female Lead and sometimes the ML has also some similarities. Both Drama also focuses on Pregnancy but it was very different on how it forms.

They're quite similar because Both Male Lead had amnesia and their Female Lead is kinda liar. Also Both Drama have some Business Stuff

They're very similar
1. Both ML are Vengeful towards the FL because of Misunderstanding.
2. The Plot is very similar the difference only in Kluen Cheewit Sathit's Lover Died but in Lhong Ngao Jun Thanakim's Friend Died.
3. Both ML's have many similarities in terms of personality
You will feel like you're watching only the same Drama

Both Revenge Drama and have a Strong Female Lead. The Plot was very different but I don't know why I feel like I'm watching Same Drama.