In both of those cases it's also important that the responses be small, since, due to pipelining requiring that responses come back in the same order as request

In both of those cases it's also important that the responses be small, since, due to pipelining requiring that responses come back in the same order as requests, you have to buffer up any responses that become ready out of order. Also, the response time for each request needs to be about the same or you have to issue requests in the right order, or, you end up potentially blocking every response due to the first one taking the longest. HTTP pipelining isn't largely unimplemented because people were lazy - its largely unimplemented because its easy to DOS a server, possibly by accident, and there are significant latency issues due to head-of-line blocking with slow requests.