Re:Monster isn't a masterpiece, but it's good and worth watching. It managed to entertain me. The premise is quite interesting, the animation isn't terrible, it's good enough not to detract from the experience, the overall narrative is good, and the development of the protagonist and other important characters is well-explored; some characters were indeed left without much development, but no truly relevant character was neglected. I would say that this anime, even with its flaws, manages to be better than many generic isekai series currently airing. I'd say it's being treated unfairly, as people are being too harsh on it, since there are many worseanime that don't receive as much negative criticism as this one.