I'm embedding the REST API in a custom application that is run on a Wildfly container. With the 7.4.0 distribution (Wildfly 8.2.1) everything worked as it is described in the docs (https://docs.camunda.org/manual/7.4/reference/rest/overview/embeddability/) but with 7.5.0-alpha4 this isn't the case anymore. It seems, that some classes are duplicated in camunda-engine-rest that are pulled in via camunda-engine-rest-core as well.
I was able to create a workaround here (https://github.com/bentrm/custom-camunda-rest-api-test), by excluding camunda-engine-rest-core and adding missing artifacts myself.
CAM-5672 may be related as it kind of describes the same problem. I'm not sure though.