It is probably overrated. But maybe you should look at other games that released in 2015. Outside of Witcher 3 - Which I haven't played yet - All of them have critical flaws to them, and they are generally rated higher than they should in critical sense. When everyone makes mistake, mistakes become more tolerable.
Contrary to what people says, I don't think Undertale is a good example of story 'writing', as writing is pretty bland and basically goes against most of good writing techniques while relying on memes and joke to carry the flow. However, Undertale excels at presenting those story in ways only game could. Showing the capability of game story telling.
However, this is just an opinion.















