I actually misread your question a bit at first - I originally thought you were asking about switching _to_ Immutable.js, not away from it. I realized that before I posted it, and decided my answer was still relevant to the aspect of "why someone might want to switch away from Immutable.js". So, in that sense it's adding context to the reasoning behind your question.

As I said, I'm not aware of specific downsides to using Seamless-Immutable. It seems like a good balance of enforced immutability in development via freezing and replacement of mutation methods, and compatibility with standard JS object/array notation and usage.