In this review it won't be intended to inform you guys of the obvious; the (seemingly not so much) common sense stuff: the protagonist of this mahnwa is a self-insert Gary Stu. What shall be discussed here is a more glaring issue though: the kind of character the fans and especially the author are self-inserting themselves into. Other people here have already done an excellent job discussing many of its vices (and qualities, e.g. the art).
Thus, let's start from the beginning: Sung Jin-Woo is a low-level young man whose mother is ill and, despite his noble efforts to pay for her medical fees through doing "dungeons", he's scorned by everyone around him due to his physical weakness. Nonetheless, he proves to have a different set of strengths; one seemingly uncommom amongst his colleagues: courage, braveness, selflessness and a pure heart. After showing us all his virtuousness, he's rewarded by his deeds with... "OPness", "Gary Stu'dom", plot armour.
He keeps up with his work after the introductionary events until we get to the 20th chapter or so, wheareas he teams up with a group of jerks who try to leave him for dead inside the dungeon. In this situation, one could've thought: "Ok, it's time for him to show yet again his kindness and mercy to those who did him wrong à la Viland Saga". No, he cold-bloodly kills every single one of them in the most cruel way he could. The plot follows like that for a few more chapters without any chance of redemption.
Instead of developing sympathy, forgiving others and maybe trying to help the world by doing more difficult dungeons now that he is so "strong" (alas, only physically), the path Sung Jin-Woo decides to take is one of seeking solace to his bitterness in others when he can only find it within himself. Once "reawakend', his death instinct is unstoppable (as in "Light's Syndrome") and the author starts desperately trying to create different convenient situations which allow him to release it without much moral consequences afterwards. He adds a ton of "garbage" human beings across his path so Sung can have an "excuse" to show his real, genuine face. Although truly hurt inside, our mangaka keeps supressing his feelings and even endorsing his actions without realising that, in real life, it would only be leading him more and more towards doom.
It should go without saying that what the protagonist of "Solo Leveling" really needs is not getting more powerful so he can completely destroy his already poor mental health, but a psychotherapist. He has good friends and also people who worry about him, thus I don't understand what's so glorious about seeing him waste his life for the sake of revenge when he could be... better than that. It's not realistic, but sad, morbid and distateful. This depiction of strength leaves brains and morals completely aside. It's indeed "Solo Leveling", but levelling down nonetheless.
