Pickles can also be time bombs, especially around python upgrades. Sometimes (ok, rarely) the serialization / deserialization of some types changes between versions of python.
Another issue is painting yourself into a corner: when you use pickles, you make it harder to either switch away from python in the future or consume the same serialized objects from any non-python (micro)service. This can delay or prevent transitions away from python that would otherwise make sense.
