DeviceKit versus SolidHAL
Missing functionality in DeviceKit
This list should give an overview of what's missing in DeviceKit(udisks/upower/...) in terms what Solid (the KDE HW abstraction layer) needs.
Solid is implemented using various backends, on Linux through HAL. Dropping HAL would, at the moment, result in severe loss of features throughout the whole KDE desktop. Developing a new backend, based on udisks/upower/... is definitely possible, as shown in this concept backend  but in the current state, DeviceKit family is missing substantial portions of HAL functionality:
- no smart polling (eg. no detection of the Eject button, no Floppy detection) 
- no notion of content types of optical disks (Video/VCD/SVCD/...)
- no read/write speed properties for CD/DVD burners, no "rewritability" flag
- no support for tape (backup) units
- no support for xD cards
Note that all of these complaints have now been refuted.