The UAD-1 will only run plugins for UAD-1, and the Powercore will only run plugins for Powercore.

The difference is that TC have made their SDK available, so there are lots of free and third party plugins available for Powercore - whereas there are only "approved" plugs available for UAD-1.

There are good arguments for both ways of working.

I think Creamware brought out a standalone DSP thing which would natively run VST synths and plugs, but I haven't heard anything of it in ages.