I started this show about a week ago, but long after the show has ended, so I have seen & read many glowing reviews & saw how folks proclaimed this drama being the best one they have ever seen.
I not for one moment have felt that way, but it took me quite a bit of introspection to -I think- figure out why.
I mean at the very least we’ve seen amazingly well acted emotional scenes, that are up there with the best of the best in Kdramaland. There is a lot of good music, romances & one amazing bromance thing, so at the very least I can understand why someone would rate this at 10/10 amazing..

Let me explain, why to me this is far worse than 10/10 & why I felt something is really wrong with this drama throughout most of the episodes. Disclaimer, I’m a rather nitpicky person. I can find logical flaws in almost anything, but that has not prevented me to give 10/10 ratings to flawed shows before.
Let’s start with a TLDR to not waste anyone’s time with my long rambling: Many and perhaps most of the minor details in this drama are either wrong or off. This pile of immersion breaking lack of attention to detail is just too tall for me.

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So here I go:

I could go on and on about -tiny or large- time travel related plotholes, but I have begrudgingly accepted them, because time travel shows always have a ton of these. (Though the level of disregard for consistency is on another level in this drama.)

Music is an important part of this drama, but the sound is completely unrealistic in most scenes. Not only does everything sound like an overly perfect studio recording, but also every guitar in the show sounds like $20000. These kids are not millionaires, yo! Sure the guitar point is very nitpicky, but I have never at any point felt like I’m listening to music made by a very talented up and coming band, or anything remotely live-like. (as in live music sounding like a live performance)

Sol Ina is just too old for this role. Yes, she acted amazingly well here & I’m happy for her success. I’m sure no one could’ve done better in her place, but I would love to see a big budget kdrama where the teens are played by at most 22 year olds. Just to reiterate again, yes she is stunningly beautiful and everything, but she's not a teen & can never pass as one. If this only happened in a couple of flashbacks it would’ve been fine, but she is a teen the entire time.
This point applies to Ryeoun as well. He is not a believable Korean teenager at any point in time. He is 25, what could he have possibly done, right?

The portrayal of disabilities in Korean media always have been very annoying to me. Here it’s probably the best I’ve seen so far from them, but there were still quite a few weird moments. Being deaf is very difficult to portray well to begin with, I don’t know. Largely because you want to keep the audience entertained, but if you as an actor do over the top emotional faces in every single scene, then the end result cannot possibly be something believable. Though of course the actors followed well what was asked of them. I can somewhat understand why they choose to do it like this. Imagine if the story was almost the same in every way, but Cheong Ah had not been deaf just an introvert, if Eun Gyeol's parents were able to hear, but lived a very hard, tumultuous life. To me it seems like most of the plot would have worked just the same. I strongly suspect that the writer added decided to go with the CODA thing, because there were a couple really big hit shows that featured disabilities. Deafness is not completely an important, integral part of the show.

Romance is only so so, I did not really feel the chemistry, nor are the romantic plots very engaging. Then again this is probably the most realistic part of the drama, because teenagers falling in love is the easiest thing in the world, no elaborate plots are needed.

The main villain is very sterotypical and boring, her actions were very silly & her end is anticlimactic. The family situation at that house makes no sense. They could have made Cheong Ah's backstroy much more interesting without opting for this whole "her father ignores her suffering" plot.

The late nineties Korea in the show is -you guessed it- highly unrealistic.

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My fellow MDL user, I’m sure you’ve noticed by now how most of my criticisms are about realism. It would be a logical conclusion that this is a me problem, since kdrama as a genre tends to be very lacking in realism & anyone who wants realism should not watch kdramas.
I agree with this of course, but if a show’s end goal is not just to entertain with comedy & sugar, but maybe to educate about disabilities, to present gut wrenching moments and to make the audience empathize with the characters, then realism becomes rather important, or at the very least a fantasy setting should respect its own established rules & just in general the writer should not insult our intelligence.
Just look at the various Reply dramas, every single one of them has a similar blend of teen romance, bromance and to a lesser extent family tragedies to Twinkling Watermelon, but they are far more reasonable in every way, the emotional scenes in those are really really great, while the comedy and romance still get to shine.

The overall presentation is a bit childish. So perhaps we should just judge this as a teen programme. That’s fair, I’ve given it some thought & perhaps watching this at the age of 14 my mind could have been completely blown, I don’t know. Most of us on MDL are adults though & with that in mind I decided to settle for a rating of 7.5, because despite everything I've said, here and there there were moments of real brilliance in the show.