Sidroe
Some people have already started having problems with the Sampletank 3 VST 3 plug, or so I have heard. I know mine was kind of buggy so I switched over to the vst 2 version and never had a problem.

 Agree totally !

U-He recommended not using VST3 version of their synths, issues with forgetting which sounds were selected when reloading and a few others...still not sure if this has been addressed.
 
http://www.kvraudio.com/forum/viewtopic.php?f=31&t=398532&sid=c0341ebcdb87929d4001b78a9d0ac408

Shambler
U-He recommended not using VST3 version of their synths, issues with forgetting which sounds were selected when reloading and a few others...still not sure if this has been addressed.
 
http://www.kvraudio.com/forum/viewtopic.php?f=31&t=398532&sid=c0341ebcdb87929d4001b78a9d0ac408

Sounds like Rapture Pro!

Steinbork shot the pooch, the fox, kicked them both into a tri-cornered cocked hat and then ended with a flourish of ballsing it right up when they came up with VST3.  A nightmare for developers; badly designed, poorly architectured, and more prone to issues (i.e. than 2.4) due to this overly complex nature (both for host and plugin authors).  You do get some fancy new bells and whistles, but really not much more than you can do with VST 2.4 already.  For example, in 2.4 you might have dedicated mono- and stereo- plugins, but in 3, a single plugin can automagically configure itself depending on the nature of the channel it's dropped into.  Not that much of a big deal and certainly not worth the considerable extra development overhead.
 
My VST3 folder is empty.

 
And vst3's have to go to the C drive.  What kind of an improvement was that.
 
 

It solves the "where is my dll" problem. A frequent subject posted here. Of course, the spec is just a recommendation about the location of the plug-ins. All Cakewalk DAWs which host VST3 will load them from anywhere and for those strictly interpreting the spec, there are directory junctions.

Apart from the U-He health warning, the biggest practical issue that I have come across is that by default, preset formats are different between VST2 and VST3. So if you cut across to the VST3 version, because it's all shiny and new, you suddenly find that your extensive library of presets is junk. This seems to be the case with Voxengo, for instance. Where the vendor uses a proprietary format, they apparently can sidestep the problem: FabFilter for instance seems to use the same format of preset.
 
BTW, I was a little surprised to find that Rapture Pro has no VST2 and comes as VST3 only.

FabFilter and Waves VST3s are great, I use them all the time.

U-He health warning was from 2013 ;-)

... but as late as September 2015 the very same thread in which Urs 'fessed up his concerns was still active on U-He's own forum. As far as I know, Urs has still not given his VST3s a proper endorsement. I may be wrong about that. of course, because I don't slavishly follow the pronouncements of developers.