First I'd like to make it clear that it's not a bad show. My praises and criticisms are dependent on the expectations I had for the show.
The good things
The idea of this drama is excellent...a show on a start-up incubator. As an engineer and being in the tech field, I really love that somebody had this idea which made have high expectations for the drama and in some ways they did a really good job to show the environment of a start-up incubator and the know-hows of starting a business and the potential for success. They also showed the risks and downsides involved in starting a business and showing the harsh realities instead of showing it a flowery path to success was very important. Also I found out that Sandbox is an actual thing in South Korea wich is really impressive.
Aside from the premise, the OSTs are really great. The direction and acting is also really good.

The bad things
One thing I really didn't like about this show is that it has too much of that "typical k-drama" drama like love triangle, rivalry, unknown past, tragedy smitten family of the main character....just too much of that drama. I''m not saying that such things always make a show bad, but there are other k-dramas to do such things. Such a dramatic plot was really unnecessary for a show with such a modern and upbeat premise. In some ways is even hindered the viewing experience. This could very easily have been a 12 episoder with only the fascinating and interesting plot revolving around Sandbox, innovation and slight comedy and romance excluding the family drama and love triangle drama then it could have been a much better experience.

It's good enough to watch one time because there are many interesting things. If you like lots of drama then you should definitely watch it, and if you don't enjoy drama all that much then still you should watch Start-up atleast once.