I just opened the Sonitus EQ in Reaper (and all the Sonitus Suite from my X1 Prod Suite install on my laptop appeared in the Reaper plugins menu thingie).
Could be just that Reaper is special in this regard (DX support) but a huge swath of the instruments and effects from X1 Production Suite seem to open in it.
I do not really use Reaper but I wanted to check it out on my laptop and I installed X1 Prod Suite just to see what would transfer over (and to see if maybe I could get Sonar running on this underpowered POS).
Various synths like SD3, Zeta+2, DimPro, etc seem to work as well but all of these working could simply be because it is a) X1 and/or b) back then I splurged on the Prod Suite (which maybe unlocked all those tools). I don't have any newer versions of Sonar installed on my laptop.
BUT this OP seems to be asking whether extra things they buy from Cake (not stuff that comes bundles with Sonar) will work in other programs.
It depends. You can buy the new CA2A and it will work as a VST in other programs but the PC2A would not (because it is Prochannel only). Zeta or Rapture Pro bought as standalones are full instruments able to work in other platforms (and even on Mac).
Anything that is Prochannel only will not work because Prochannel is only available in Sonar.
Just look at the product description in the Cake Store listing for more detail or contact customer support for more info on specific products.
Cheers.