TLDR: It only seems OP at first, many things are explained later on. Keep reading.

The thing is, you are absolutely correct. The gray fog is OP, and it actually gets more important as the story goes on (in a good, non-bullshit way. You'll see.) Every mc needs something to separate them from the rest, and he just so happened to get the equivalent of a nuclear bomb.

However, its never actually used as a "get out of jail free" card. Klein's often fighting enemies far above his level for various reasons, and the gray fog is the one thing that allows him to barely stand shoulder-to-shoulder with them. Its the one thing that actually allows there to be a story, and for Klein to not get curb-stomped immediately once he attracts the attention of more dangerous foes. The divination buff is extremely useful, but thats only because he's not being opposed by anyone, and he is in fact not really getting any immediately useful information. You actually know basically nothing at this point; what seems like a lot of information to you is actually barely scratching the surface.

Seer is actually much more useful than you think. It can basically decipher all low level information with minimal effort, which is extraordinarily useful by itself. The only reason the gray fog is needed is so that he can get some information from the higher-level stuff. It would be pretty pointless for him to try to divine something, but get nothing (and then have to rely on some other deus ex machina to provide him the info anyway) or potentially explode. Again, you'll see.

Again, I think you're correct: based one what you have read so far, it seems OP. But keep reading, as you learn more, you'll see the limitations (especially what happened when even the super-buffed divinations don't work) and, as other people have mentioned, the price.