Well I get why they don't like the ending because the ML and FL didn't really have that "romance", if your a sucker for romance, you'll hate it. They kissed other people but they don't have that moment for themselves.
But it was perfect for me, because I like her more with the other guy. Their chemistry is top notch. Like Lee sieon and Lee siwon are made for each other, her liking parang while he likes her but he doesn't really show it(my fave tsundere). he's a popular but lonely guy. In the mid story, I want them together when I noticed he liked her. I thought she would surely end up with Parang and I feel bad for my ship because I thought it would surely sink but guess what?? They kissed and even have more romantic moments together, he was so determined to risk that friendship to be with her, thats when I got that 2nd Lead Syndrome for the 1st time. I dont know if they got together but I like it that she didn't end up with Kang Parang, but in the end he was holding out his hand and she was about to receive it, it's unclear but that's fine. It would ruin the story if she did end up with parang. It looks like she got used to it more than seeing him as a man.