The only problem I have run into with VST3 is the u-he synths.  Based on a posted FAQ I read a while back, apparently Urs is still wrangling the code for his VST3 plugins. He recommends sticking with his VST2 until he gets it worked out.
 
https://www.u-he.com/cms/faqs#know-vst3-issues
 

Hi. I have installed the Vst3 version of all the plugins (exept SPAN, that has caused me some issues) that have that option since it became available. 
Welcome to the true power of Console 1 
All the best.

bapu
I'm so happy with VST3 I can hardly wait to see what VST4 does.

It'll probably do the same thing - just with a monthly subscription and patch Tuesday auto-updates

You will have to wait until VST5 to be able to use stock price trends and weather forecast as morph source ... and rumour has it that VST6 will then be bidirectional as regards morph and previously named external sources ...

With some VSTs, your only choice is to use the VST3 version. For example the VST2 version of Diffuse by Surreal Machines causes Sonar to hang, but the VST3 version is absolutely fine.
 

Another VST2 that I recently noticed had problems was the Oril River Reverb.  https://www.kvraudio.com/...river-by-denis-tihanov
 
The VST3 seems OK, but in my use the VST2 resets to default parameters when you re-open a project.

It seems that that problem is not particular to Sonar. It also appears to happen in other DAWs:
 
http://www.kvraudio.com/f...amp;start=225#p6807708