Well, can only say I found doing a full sync (of several hundred Gb) takes maybe 7 hours with OF and more like 20 hours with emcore rockbox. Definitely found to sync 300gb or so takes all night and much of the next day, but the one time I've done it with OF it seemed to be about 2-3 times as fast (only taking all night!).
Its true that I haven't found rockbox usb actually falling over during a sync since many Rockbox versions ago. Either I'm using more reliable ipods or they've improved things. But early on in using rockbox and modded ipods it seemed to be very unreliable. It definitely would drop the connection quite often in the middle of syncing.
As for subtle transfer errors, really can't say I've noticed them myself, I'm only going on what has been said by others in this very thread (or maybe in the ipod modding thread). Doubt I could dig up the posts now, but I've seen it claimed that on checking hash codes or signatures or whatever its called it turned out files were not copied perfectly.
On Rockbox's own site its been acknowledged in various thread comments that the USB handling isn't entirely reliable on the classic (it is still officially 'not working' after all!).
(All of which is why its great that dual-booting seems to be pretty much working now, if someone could just find a solution for the C++ errors mentioned above! And kudos - whatever kudos are - to those who have made such progress on dual-booting)