I have some old favourites which I do not ever see updating to VST3 (the author appears to have abandoned them). These are essential and I see myself using them for at least another year unless a competitor comes up with a modern VST3 alternative.
 
Will I run into issues if I use both VST2 and VST3 plugins in the same Sonar project?
 
(I don't have Sonar X3 yet so I can't try it myself)

Yep. I wouldn't mix 32 & 64 bit it looks like, but vst's 2 & 3 are good.
 
 Brian
 
Edit: It's safe, title and content ask differently!

I mix a few 32 and 64 bit plugins along with VST 2 and 3 without any issues of but there is no VST 3 bit bridge so any 32 Bit plugs are all VST 2s.

The major issue seems to be with UAD and Waves plugs and not mixing VST2 and 3 on a per fx box case.

I'd recommend experimenting along the lines I've mentioned and if you run into any problems then patiently wait for Sonar X3c which is due out before the end of the month and promises quit a fe VST fixes and enhancements.

Prochannel modules are in 2.4 VST format and work fine with VST 3 plugins in the FX bin. 
 
The only problem I have heard of is with mixing Wave's plugins that are 2.4 and 3 VST. 

That's all I needed to know. Cheers!