You will still have the process engine and all services created and autowired. But you won't have a process application registered and thus won't be able to use webapp with forms (because form-key resolvement requires the engine bound to an application with a servlet-context),
If you just want to run an engine and maybe provide a rest api, you are still good to go.
But you should really consider just touching the empty marker META-INFO/processes.xml and go on with the annotation. Default of processes.xml is to scan for deployable resources, so the behaviour is like autodeploy.
I know that using an extra xml is annoying, but it's the camunda way and it works. There already is an issue on the board to drop the processes.xml and allow configuration via yaml, but it proved to require changes in the core platform, that why for now, I stopped working on it.