Hi Oliver Gierke, I hear you and have had some issues as well (maintainer of Hibernate Search).. we need to improve on this, although it's not trivial to extract some actionable suggestions from your points.

Some of the things you list were unintentional (bugs), and to improve on those we already discussed on the mailing list that at least a CR should appear also for minor versions before the release. Lacking this though, I think it's fair for "downstream" projects like ours to raise any issue and wait for the next bugfix release rather than having to rush out a compatible version.

For example Hibernate Search can't upgrade to Hibernate ORM 5.2.0.Final because of issues with org.hibernate.Query .. my bad as I could have noticed earlier but I've been busy on other matters. I fixed the issues and will be able to upgrade to ORM 5.2.1.Final once it comes out, however I'm not sure if it's the same issue with org.hibernate.Query that you are referring to! Please check and raise follow up issues as needed.
I suspect Steve was expecting us to more closely test against nightly builds, but this hasn't been very practical for the Hibernate Search team (for example), as e.g. when internal APIs break we can no longer have CI test both branches with a single source set.

Regarding the Java 8 compatibility, that one was discussed on the mailing list and there were no objections. I can see how someone would not consider this "proper semver" but to be fair I'm not sure I agree. Might be nice to have you on the mailing list so that when such tradeoffs are debated we can also consider your POV at a more timely moment.

Re the 5.1.1 release: I asked about one as well this weekend: we might see one this week according to Gail - who is in charge of maintenance of older branches. Steve just explained me that the core ORM team usually works only on releases of the latest branch, so when there's need for a maintenance release of an older branch we might need to ask for one explicitly: for people like you they'll likely do one, but don't take them for granted as there won't be releases unless there's demand or volunteers for making them happen. I hope this helps!