No one expects the spanish inquisition and no developer expects that parsing data by itself can lead to a security weakness, but still PHP is always happy to help you have an interesting life.
So what is the problem with json_decode? the problem is that with a properly crafted JSON structure an attacker can force a very slow parsing of the JSON and hang up the CPU. The weakness comes from the way PHP internally stores arrays in memory that in the normal use case is very fast to access and retrieve data, but the worst case is horrible and json_decode by default tries to create an array from a JSON string, Not sure if the object option of the API is any better.
Mitigation? It doesn’t seem like you can guess anything about the content/structure of a JSON string without actually parsing it therefor it is probably a good idea to check that the length of the input “make sense” before processing it.