The "toughest" I've found is Interlaced 1080/60 material when in HQ mode on a non nvidia PC. My Q6600 is not up to this task,
I think there is likely some variability in this though.
My HTPC is a Q9550 @ 3.5GHz. For MOST video files, ROHQ is fine. However, when I play a few of my high-bitrate BluRay rips (13-16mbps or higher, usually, @ 1080p), and also H264 compressed TS files (16mbps @ 720p) that I recorded myself, I get stuttering and dropped frames. I'm bitstreaming and I haven't tried messing with refresh rate changing yet.
Most other content plays fine in ROHQ.
That system has a AMD Radeon HD 6870. I wonder if madVR has some sort of "preference" for nvidia hardware, simply because that's what he uses to develop it, and there might be something he does that performs poorly on AMD hardware that he doesn't know about?
On the other hand, my basement system is a Core i5-750 (Nehalem) @ 4.0GHz with a weaker GPU (AMD Radeon HD 5850) and it is smooth-as-silk, so...?? In my experience, ROHQ likes Nehalem or newer CPUs, though I don't have a modern-enough nvidia card to have played with LAVCUID yet.