Right now the answer is I’d say around half work. There may be more that are unlocked in the future.
So the ones that for sure DON’T work are any of them that are DXis like TTS-1 and the Sonitus FX, since Cubase doesn’t do DXi. There is a DXi to VST adapter out there if you really need that. Also any of the ones that are 32-bit only don’t work like Dropzone and RXP since Cubase has no 32-64 bit bridge, you’d have to get something like jBridge. Also none of the pro channel modules work, nor can they since they are customized. Have to have a non-PC version for them to work.
Past that, some work, some say “Cakewalk only” and some can be fixed.
In the “can be fixed category” there is the Overloud stuff (TH2, Breverb, REmatrix) the AAS stuff (Strum, Ultra analog and such), and TruePianos. All those companies have offers where you send them proof of your Sonar ownership, they’ll give you unlocked version.
In the “works” category is most of the older Cakewalk plugins like Boost11, Channel Tools, LP-64, PX64, VX64, and so on. Bifilter also works, as does the new LP EQ and LP MB. Rapture Pro also works fine, if you have it. So does CA-2A.
Right now the biggest one that doesn’t work is probably the Adaptive Limiter. Cakewalk could fix that, and maybe they will, but for now it doesn’t work. Also none of the Nomad Factory plugins work, they all complain about needing to be in Cakewalk.
All in all it isn’t too bad, particularly if you go and get some of the 3rd party ones unlocked. Plus Cubase Pro at least comes with highly competent replacements for most stuff. You won’t at all miss Cakewalk’s old convolution reverb when you have REVerence, and Halion Sonic SE is a competent replacement to TTS-1 and so on.