I was using all AU plugins for my item FX chains with no problems. I had to start using VST versions of UAD plugins because for whatever reason, when you bypass AU plugins in REAPER, the UAD DSP is still taxed. The same goes for playback and rendering, even if an item in the timeline isn't needed for playback or render, the DSP is still taxed which means that in a mastering context it's easy to max out the UAD DSP when using AU versions but when using VST3, the UAD DSP is barely taxed.
So, I am using UAD as VST and the rest as AU.
The first time I move to the UAD/VST GUI when audio is playing back, I get audio playback disruptions for a second or two.
Not a huge deal but it would be nice if I could navigate to the UAD/VST GUI with smooth playback. I don't know if this is UAD specifically or all VST/VST3 plugins when mixed with AU on a item FX. I also don't know if this has any relation to having a track in record enable. I'm basically sending the track with the item FX out to a hardware output, and capturing back to a new track which is how I am monitoring all this.
Here's a screen shot of a typical item FX chain for me: