The warnings, other than symbol GC in MRI <= 2.1, refer to using Oj in `object` mode: marshallings arbitrary Ruby object graphs to/from JSON. This is similar to pickle in python. This mode isn't what you would be using at the boundary of something like a http/json api.
>Taking an arbitrary String from a user and evaluating it is never a good idea from an unsecure source
In that case, how it is possible to implement JSON post on that for example?