Tchaikovsky is uneven, as are Banks and Reynolds. Lem is just different. He’s rarely a hard sci-fi writer and some of his books are barely sci-fi. More like Voltaire (Invincible is sci-fi)
In fact quite a lot of Lem's writing is not sci-fi at all, period. Hospital of Transfiguration is an absolutely mandatory read for any fan of Lem, even though it will leave you depressed for a week afterwards. And memoirs found in a bathtub is just a masterpiece of absurdity, it has a very good English audiobook actually if anyone wants to try.