Short papers are just fine. Long documents, as long as you read them mostly sequentially, are IMHO just fine as well. But trying to read something where you need to flip around a lot is painful.

One of the use cases I had in mind when buying rM was reading stuff like IEEE 802 standards, programming language standards, hardware references, etc. But with these documents, the table of contents itself can have 10s of pages. The tools for searching and moving around are there, but the high latency of the e-ink makes them frustrating to use. And hyperlinks in PDFs are not active. And you can't bookmark something to create your personal collection of pages that you often need, which would somewhat alleviate the abovementioned frustration.

I use rM daily, but just not for reading manuals. It's just not a good use case for the device.