I'm not quite sure what you consider "not mainstream" or "underrated" considering Prison Playbook has 37,000+ watchers and is one of the highest rated dramas on MDL, but I'll do my best. The following dramas all have less than 20,000 watchers on MDL and I thought they were good.
Six Flying Dragons - This one is great! Highly recommend.
The Hymn of Death - It could have been better, but it's a good story and is not long, so I'd say it's worth watching.
White Christmas - I watched this a long time ago, but I remember enjoying it and thinking it was very good even though psychological thriller is probably my least favorite genre.
The King in Love - This one wasn't great, but I think it's better than the rating might lead you to believe. Definitely flawed, but it had some aspects that I enjoyed a lot as well.
The Crowned Clown - I would have thought this one was more popular, but apparently not. The story is very standard for a sageuk, but the execution is above average.
Familiar Wife - This was probably longer than it needed to be and it was sometimes a bit frustrating, but I think the themes were nice.
These are some old dramas that were definitely mainstream in their day (all had high ratings in Korea), but I guess they're not popular because they're old: Warrior Baek Dong Soo, Queen Seon Duk, IRIS, Iljimae, Jumong.
I know most of these are historical, which might not be the genre you're looking for, but I hope something helps. :)