I do not have Nucleus, but there was several discussions about it in this forum.
Facts:
* Nucleus does not not have own software for Sonar, it works throw MCU plug-in
* SSL blame Sonar/Cakewalk when it comes to problem
Rumor:
* MCU emulation on Nucleus side is not accurate and Sonar MCU plug-in does not play well with it
My opinion:
* MCU plug-in is open source. SSL want significant money for Nucleus, but they do not want spend money for 1-2 days programmers work to fix all problems in Nucleus+Sonar integration.
So, the only options are MCU Pro + XT (limitations: no synth control, no ProChannel control, manual configuration for FX controls in special file) or VS-700 (discontinued but still works).