You're unlikely to find many modern MP3 players that act like a real slave drive. This seems to be becoming less popular for manufacturers to do.
The regular lockups in the retail firmware might be a concern though. That probably shouldn't be happening, and if it's caused by faulty hardware of course Rockbox won't address that. Rockbox is a replacement firmware, so doesn't strive to fix problems with the original, but rather just is itself, with its own advantages and problems.
Rockbox has good crossfade and a very good random shuffle, allowing you to easily create playlists of any segment of, or all of, your music, and shuffle it all, or play individual folders (shuffled or unshuffled) and then move from folder to folder (sequentially, or randomly).
In regards to development, Rockbox is one piece of software that runs on multiple players, so really, there is no real point at which development stops for particular models.
There are some concerns with the H10 port: Specifically it has poor battery life, and does not have its own USB mode (which means you still have to depend on rebooting to use iRiver's USB mode). As well, I believe the radio is not implemented yet. This is due, in part, to a lack of hardware documentation, and while the situaiton on all three of these fronts will improve, these factors may dissuade you.
That being said, I think you may find it difficult to find an MP3 player with the depth of features that Rockbox offers on any target it supports.