The main difference between iPod and "other DAPs" is that you load your music to iPod via iTunes, a desktop software, which deals with file formats, tags, artwork, library indexing, while with "other" DAPs the player software does it all itself on the device itself. That makes the most of limitations and causes most of problems.I do wonder why other DAP's have such limits when the iPod can read up to 40k
Another limitation is the software quality. Apple sells millions and is able to invest more. People like rockboxed devices because Rockbox is developed by community for years and thus is more robust.
It should not be a problem with DX80 to fill 200GB card with properly tagged MP3s or AACs, small size or no artwork. You can swap cards, but then either wait for re-scan, or you are limited by playback from folders. Also, there is not a problem to play them from 3TB USB HDD via OTG (but no library pre-scan, by folders only) - I know because I tried.
But the question in general "may I put whatever I've downloaded from Internet, with print size artworks, and expect fast scanning, easy access and flawless playback", the answer in NO.