With MPE on the Hydrasynth, there are three messages sent in an MPE channel : Note-ON, Channel Aftertouch, and Note-OFF. In the case of the Explorer and the Deluxe the Note-OFF can have a Non-Zero velocity value, the HSK and the HSD are in channel but both have a value of Zero. This is a hardware limitation.
( I do not own a Explorer or Deluxe — poor me — so i have not tested the MPE note-OFF velocity feature on those two, so anyone who’s tested this, please correct me ! )
Other MPE controllers may have more parameters that are passed in the MPE channel than with the Hydrasynths. That’s part of the trade offs you must evaluate when deciding between your options.
If you want to use the Channel Aftertouch, or Note-OFF in channel MPE messages for other functions you map those controls in your MPE synth or software.
For more options for MPE controllers see this list.