Just like Itaewon Class, Start-Up lost everything after episode 10. To be precise, episode 13 destroys everything this drama should have had. In the first week of the broadcast, Start-up hit a rating of 9.0 in MDL. It was high and it was NOT overrated. The drama was really good, thus many anticipated the next episodes.
When it came to the end of the broadcast, the rating fell down to as far as it reaches: 7.9 (when this review is written).

Why does it flop?
1. First, get to know the meaning behind the title: Start-up. Establishing and developing a young company founded by one or more entrepreneurs. Yes, this drama should have been a story about Dalmi and friends fighting to become a top-tier entrepreneur who runs a successful company in the end.
2. Everything about developing a start-up company is the MAIN VALUE of the drama. I believe the writer has researched well for this drama so that it really provides new information about technology and business.
3. From the two points above, you now should have started expecting an ending where Dalmi and friends have successfully established a company from the ground.
4. Everything goes smoothly before episode 13 destroy this drama by throwing out the main value (mentioned in point 2). It skipped like the whole story. Dalmi and friends were struggling in the company, but episode 13 skips it.
5. EPISODE 13 shows you how Dalmi and friends now have successfully lived their life as an entrepreneur WITHOUT showing you THE PROGRESS.
6. I believe the writer/director prioritized the romance instead of the business aspect of the drama. So, when they feel the "fan-service" i.e romance scenes are not enough, they add it everywhere in the part within the episode.
7. Yes, so many unnecessary romance scenes that could be cut for the solidness of the plot's sake.
8. I believe the writer was in confusion about whether s/he should pick Dosan or Jipyeong as Dalmi's boyfriend as the majority of the watchers tends to love Jipyeong.

However, the drama was saved by:
1. Duo Chulsan and Yongsan. Their comedy is the best in this drama.
2. Chulsan and Saha's love lines. This better than the main roles'.
3. Touching family scenes. Ngl, I cried when Injae met her grandma.
4. The complexity and well-researched of the terms about the computer, technology in general, and business.
5. Interesting story in the first half of the drama. It successfully brings the drama far from boring.
6. Solidness plot in episode 1-12. Every event has a connection to other events and it triggers another conflict.
7. Grandmas: CEO of Sandbox and Injae's grandma.
8. Suzy.