Although the plots are very different, they are very short dramas. LGBT+ romances. Quickies (can be considered movie-length). You don't need to think much. Just enjoy.
Both are short BL shows with main character(s) associated with gangsters. While red balloon differs in that the main characters are teens for most of the story, and it’s less intense than long time no see, both have very heartwarming and convincing relationships between the main characters, and interesting side characters.
The similarity of these two is the secret love of an assassin. Long Time No See is about two assassins falling in love with each other not knowing the identity of one another as enemy. Nevertheless, against their superior they've chosen love over their assignment. Whereas Bangkok Love Story is about an assassin develops feeling for his assigned target.