hey Frederik,
the prospective search API has been in Alpha way too long, i agree. we're currently working through options on what to do with that particular API moving forward. the functionality is very useful (IMO), but the underlying integration between the backend service (internal to Google) and Cloud Datastore is not ideal longer term. i would suggest holding off on using this API for any production workloads at this point.
related, i get that there is some cruft in the overall API set; some APIs lingering in Alpha and Beta for months to years. we're in the process of looking deeply into the situation. look for an update in the next 1-2 months.
-- Chris
Product Manager, Google Cloud Platform