One of the things I've noticed about any Rename operation that involves moving files - it updates the database to reflect the "intended" new location of the files before it has started to move them instead of waiting until the files have been placed in their new location.
All of these things are side effects of the fact that Rename, Move, and Copy operations in MC are tagging operations, and are done through the regular tagging engine. The RMCF tool is just an automated method of changing the [Filename] tag. You could change it yourself by hand, and it also moves the files (just like RMCF in rename mode).
Though I will say, two things: I haven't ever actually seen it not show an error message eventually if you let it go long enough. However, it will also wait on stuck files a LONG, LONG time. If a file remains in-use, the MC tagging engine (like any tagging operation that can't write tags to disk) will just keep trying until it succeeds. If you kill it while a RMCF is running, thinking it is done, that's where you'll get into trouble. I'm assuming that's what happened to the OP. Killed it, or rebooted or something, while MC was still trying to move the files (perhaps because some were read-only, or not writable, or otherwise stuck and so it was never going to finish).
The second thing is that I agree it could use some work. A progress bar, cancel button, and completion queue, similar to a Handheld Sync would be my preference. It could still process them as a tagging operation as it does them, but because moves and renames are often a much higher "latency" (when moving from disk to disk, certainly) than a normal tagging operation, often long-running, they really need special treatment. The Status Bar status is not appropriate for them (and doesn't even show constantly, as you switch around Views). It needs a progress dialog, and the cancel button, if hit, should prompt whether the operation should be cancelled and left as-is, or rolled back to before the operation was started.
And, while you're at it, it really needs to handle overwrite collisions itself, rather than throwing that to the OS. As it is now, it too often pop-ups constant "Are you sure?" dialogs if it has to overwrite files on disk (usually when moving sidecar files and other metadata files). It is impossible to get it to just "continue", or even cancel, the whole batch as it is because the OS dialogs get called 1-by-1 and not in a "batch" (so checking the "and don't ask me again" checkbox doesn't work because it only applies to that one move, and not the next). MC should, instead, handle this internally prompting the user as needed with
its own "keep going and overwrite, create a copy, or cancel" dialog with a "and apply to the rest" checkbox, that actually works.