Pulp

There is a serialization issue related to others (like #1527 & #1535), but I think there's also a client-side issue. Including the 'unit' field in 'filters' makes this look like a unit association criteria, but search views are expecting search criteria.
See https://pulp.readthedocs.org/en/2.7-release/dev-guide/integration/rest-api/content/units.html and https://pulp.readthedocs.org/en/2.7-release/dev-guide/conventions/criteria.html for more detail. If these queries were working in 2.7, I don't know that they were supposed to; the documented API for 2.7 doesn't support unit association criteria in search views generally, and the content unit search views at pulp/api/v2/content/units/<content_type>/search/ are documented as accepting search criteria.

So, after my serialization fix makes it to 2.8 (PR coming soon, I plan to clean up https://github.com/pulp/pulp/pull/2343 and put it in there since it's a related two-line fix), the request criteria should be more like

{
    "criteria": {
        "filters": {
            "id": {
                "$in": [
                    "RHEA-2010:0001"
                ]
            }
        }
    }, 
    "include_repos": true
}

instead of

{
    "criteria": {
        "filters": {
            "unit": {
                "id": {
                    "$in": [
                        "RHEA-2010:0001"
                    ]
                }
            }
        }
    }, 
    "include_repos": true
}

(remove the 'unit' level)

I haven't checked (yet) to see if those criteria are coming out of pulp-admin. I'm going to have a pretty good laugh if they are.

Sorry about this but in my effort to clean up the json I put the wrong params in the first description. Correct ones are below matching your updates.

For errata it is

POST request to /pulp/api/v2/content/units/erratum/search/   with params {"criteria": {"filters": {"id": {"$in": ["RHEA-2010:0002"]}}}}, "include_repos": true }

For package categories

POST request to /pulp/api/v2/content/units/package_category/search/   with params {"criteria": {"filters": {"id": {"$in": ["all"]}}}}, "include_repos": true }

For package groups
POST request to /pulp/api/v2/content/units/package_group/search/   with params {"criteria": {"filters": {"id": {"$in": ["bird"]}}}}, "include_repos": true }

For Distributions

POST request to /pulp/api/v2/content/units/distribution/search/   with params {"criteria": {"filters": {"id": {"$in": ["ks-Test Family-TestVariant-16-x86_64\"]}}}}, "include_repos": true }

After removing a pesky backslash from the distribution search, all of those are working on my feature branch. Woot. :)