It's very sad (in the beginning) and very sweet.
I'll start off by stating my feeling: do you know that feeling when you watch someone constantly degrade themselves and you can't do anything to stop it? It's a frustrating feeling. I get that same feeling while reading this manga.
It makes perfect sense that FL thinks of herself as someone horrible because of the neglectful environment she grew up in, and is in fact quite realistic because commonly, people who grew up in such a horrible environment would have low self-esteem. That is absolutely okay, and I understand it's a coping mechanism.
Some recap of the storyline (contains spoilers): FL is treated as absolute dirt, and then is married off to a seemingly super cold elite ML. Some misunderstandings happen, then the nice old lady (close to ML since he was a child) communicates between two of them and solves the miscommunication. ML warms up to her as he sees her kindness and gradually learns about her past. FL very slowly grows confidence (yay character development!). That's a very sweet relationship. Then, SPOILERS... Her family falls to ruin. Take that, FL's crappy family! I hate FL's sister and I was satisfied to see the retribution. From the light novel, FL discovers she has the power everyone wants.
But all in all, as I read the story, I didn't feel anything much. Just, meh. It felt like the author was just trying to make her suffer as much as possible and then power up everything to be in her favour.
My review is subjective. There are many other great reviews that explain why they liked it, and I suggest you read them (for example, this is a good review: https://myanimelist.net/reviews.php?id=382926) too to form your own opinion on whether you want to read this. I guess for me though, this just didn't click.
Apart from its setting, it's really average and cliché. I did enjoy it though, and if in the future I stumble upon its completed version I'll gladly read it.