Well right now the X99 platform with DDR4 and especially the 900 series Nvidia cards perform the best with GPU acceleration especially in Adobe. Cuda still far out performs open cl and has less issues since the development time has been far greater on Cuda. I would suggest that especially since OSX updates have been breaking open cl for many applications with less performance as well. Just because many stay Mac as a preference doesn't mean it's the best solution or without issues. Right now a nMPro cant perform anywhere near an X99 system until Apple updates their platform.
I run a 6 core mac pro with 32gb ram. I've never had a problem with speed in Premier - I use it frequently. I'm cutting raw R3Ds in a 1080 timeline and 4k dslr footage no problem. All high resolution with lots of filters. I can work in a 4k timeline at half res, 1/4 with filters. I'm sure an equivalent priced PC will export faster, but I usually go make a cup of tea or sandwich when I'm rendering anyway! What I have had a problem with is plenty of crashes. I edit a lot of 5 minute ish videos and I'd say on average I end up with around 5 "copy of…." recovery versions. I've never lost any work though - the recovery has always been solid so its not a big deal since it takes about 20 seconds to reload premier after a crash. I'm not sure if this is mac specific or if Premier is buggy. I get the impression that plenty of people (mac and PCs) get crashes in premier fairly frequently. I've had to spend a disproportionally long amount of time on support with adobe (not just for premier) than for any other software I run. Kind of annoying, but not a show stopper by any means.
I think one thing that is more openCL specific is issues with LUTS and Lumetri. My system slows down to totally unacceptable levels. I have to work in Software mode to get any kind of playback. But I think PC users have had some strange issues as well.
Anyway, I think the general consensus is that if you're going to be a dedicated Ppro user then use a PC. You'll get better performance with more up to date technology for sure. I went with a mac because I use FCPX a lot (a much much more stable and enjoyable experience) and I'm a freelancer who is often asked to work with FCPX and still fcp7 for clients. Also 90% of my clients want pro res deliveries. There's a lot of reference to having a "balanced system" on the forums and I personally think that should include your general workflow and lifestyle, (incidentally the small form factor and portability are pluses for me as well), not just the hardware.
Finally if you are going to go mac, never update unless you absolutely have to. And never until the updates been out a while. You're at the mercy of apple and they don't give a cr*p what compatibility issues you might have.
A PC system setup right with Nvidia GPU does not crash in Premiere often if at all. The reason is Cuda is still far more stable right now especially with memory management in Adobe. Also Cuda works fine with Lumetri. There have been issues with that on the open cl side since it was added.