RMC does NOT move subfolders and my experience with ancillary files has been lukewarm at best.
Correct. It moves the files that you select using a View or Playlist. The files can be from any sub-folder, which MC knows, and RM&CF puts them into the folders and sub-folders that you tell it to, based on the rules that you set in it. Which means that it can omit sub-folders or change the folder structure into which files are placed if you use different rules to the original structure. It doesn't move empty folders of course, as it only moves files. So you can lose an empty folder in a structure, but not an intermediate folder, empty or not, if the rules require it. Plus it does update the MC library at the same time that the file is moved, so it isn't lost and an extra step isn't required.
Ancillary files can be a bit of an issue, but a lot has changed in recent versions. For my uses the ancillary files I do have get moved, because I don't have any non-standard ones.
Of course, if just changing to a larger drive, which is for the sake of discussion drive M:, then cloning or copying the whole file structure from the old drive to the new drive using tools outside MC, and then setting the new drive as drive M:, is probably quicker and will produce the exact same file structure, and so work with MC straight away when it is restarted (because it shouldn't be running when doing such a copy).
Otherwise, I use it a lot with no issues, mostly moving files from a full disk to one with more space. Essentially archiving Movies, TV shows and recordings from their designated TV recording drive to another drive, if I want to keep them. I actually use it with standardised templates (Presets) to standardise the folder structure and naming I use I use in my library, for those files that aren't already in standardised structure and format.