First, this drama wasn't bad. It was good, but I feel like they missed out on some great opportunities. I'll start out with the positives, and then go onto what I think they missed out on.

Pros:
Child Story: Personally, I love the child story more than the adult story lol. This and Queen for Seven Days rocked their child stories, and if the whole story was the children, I'd be okay with that, prefer actually lol. The child actors had great chemistry, and I loved their story, and especially how it parallels when they're older. Da Reum Nam (Jae Chan younger version), especially did well, and I look forward to his future projects.
Music: You can give this drama a 1 star, but the music was on point. I haven't seen such an amazing ost from a drama, in such a long time. Eddy Kim's ost was especially amazing.
Transitions: this drama does it's transitions quite well. They made the moments much more emotional, and I really applaud their style. There was one transition/parallel scene in the last episode that nearly had me in tears (which is good for me)
Acting: I won't say it's amazing, but it was certainly good. Especially the last episode, there were some emotional scenes with Jong Suk that were really done well. Also as regard to Suzy's acting, I'll probably get hate for this, but I feel like most of korea's actresses aren't amazing. They're good, but there are very few who I would actually genuinely compliment and call great. I feel like Suzy did an average job. I didn't cringe, and she delivered her character well, so I'd qualify her as average. That's just me though.

Cons:
Predictability: this drama is hella predictable. Oh. My. Gosh. And if it focused more on character development instead, then that would be fine. But it doesn't. Frankly, our two main leads aren't that interesting, character wise, and on top of it, the plot is predictable. Now some may say, I should've known it would be predictable. But Signal wasn't predictable. They could've just been like "oh we have walkie-talkies and can fix all past crimes, so every episode is a win" but it wasn't. Signal definitely had moments where their characters failed, and where their characters succeeded. Causing real suspense. There was no real character development, and there wasn't even suspense, resulting in boredom.
Side Characters: I love our side characters. Woo Tak, lil bro, dae gu, and inspector choi. They all have interesting characters. Woo Tak and his internal debate of what it is to be a police officer, and if he has the right. Lil bro and his school problems, and learning that being a good friend is more important than popularity. Dae Gu with his father, and the struggles that comes with, and how he finds a friend in lil bro. Finally Inspector Choi, and how he needs to forgive himself. Honestly, I thought all of their stories were much more interesting character wise, yet they got hardly any screen time. Definitely a star off for me.
Kim Soo Hyun: why? She has an arc, a romance, a sismance, great chemistry with cast, and a great story. Yet, they just cut her off. Why put her there to begin with? Honestly, everyone wanted her to stay. For a guest role, she had such a great presence on screen, and it just left me disappointed that she didn't stay.
Main Characters: I mentioned this in side characters. They just are too perfect for me. I know they're always going to succeed. While I liked their characters, they certainly weren't memorable.

Overall, this drama was good. It just isn't something I'll draw back on. The writer missed out on a lot of opportunities with suspense and side characters. It felt too safe for me, and no chances were taken. Hopefully, next time the writer can make a better drama, as I know he is capable of.