I have labored with Sonar X2 for quite some time. My complaints are not about its complexity, but quirkiness. There are times that the product stability seems compromised (the other day it disabled the MIDI input to a channel for no obvious reason).
 
So I wanted opinions about the desirability of upgrading from X2 to Professional.
 

• Is the product stability improved, and by how much?
• Will I lose any capabilities by using Professional and not Platinum?
• Are their functional improvements, such as better handling of latency and dropouts (two things that plague me)?

 
Also, any insights from people who have made this leap are welcome. What did you stub your toe on? What worked magically better?

Stability issues for me were a bit quirky in X2 also.
X2 compared to Platinum (rock solid compared to X2) IMHO. Like night and day, almost like two totally different DAW's (no comparison).
I don't have any experience with professional as I have always been in the Producer product range from the very beginning starting with Sonar 4 Producer.

X2 was not the most stable version. Platinum is very stable overall. Professional should be very stable too. But I have not tried it so I have no first hand knowledge. 

X1 was a mess, X2 was a slight clean up, X3 was finally stable. Platinum is better. I wish that the products were as solid as Producer 8.5. There's a learning curve from X versions to Platinum, but not too significant. I don't suspect you'll be disappointed by moving to Platinum. 

Try the demo of Platinum here: https://www.cakewalk.com/Products/SONAR/Try-SONAR#start
 
Then look at this feature comparison for Sonar editions. 
https://www.cakewalk.com/...s/SONAR/Versions#start
 
The biggest differences that I see in Professional, for FX, the Boutique Suite FX is limited to just TH3, and the Pro Channel options are somewhat limited.  But I would think that some of those were provided in X2, and should carry over.  As far as instruments, there's no AAS Session bundle (Strum, Ultra Analog, Lounge lizard).  The only thing missing from core features seems to be Drum Replacer, Vocal Sync, and Theme Editor.
 
 

8.5.3 is still the bar by which Cakewalk should hopefully aim to achieve one day again.  That version of Sonar was bulletproof.  I even had a project that for some reason kept crashing Platinum, yet would open with no problem at all in 8.5.3 so in my opinion, that is still the most stable version ever. 

Platinum's not bad.  I get a few crashes a week with it, working regularly.  Not as bad as the X series, not as good as 8.5.3.  In retrospect I end up restarting Platinum more for unfixed bugs than crashes.  Things like the disappearing now time bar, or when splitting clips stops working.

Ionian
8.5.3 is still the bar by which Cakewalk should hopefully aim to achieve one day again.  That version of Sonar was bulletproof.  I even had a project that for some reason kept crashing Platinum, yet would open with no problem at all in 8.5.3 so in my opinion, that is still the most stable version ever. 

Platinum's not bad.  I get a few crashes a week with it, working regularly.  Not as bad as the X series, not as good as 8.5.3.  In retrospect I end up restarting Platinum more for unfixed bugs than crashes.  Things like the disappearing now time bar, or when splitting clips stops working.

Its funny I don't see it that way. It was stable but not as stable as Platinum has been. Plus Platinum has been updated every month 8.5.3 was the last update for the 8.5 version. I think it depends on so many factors there can never be a true comparison. 

Well, I was hopeful, based on comments above. Thus far, it has been a farce.
 
Basically, Sonar Professional bombs during installation. A series of DLLs won't register and the program will not start.
 
I bounced the errors back to Cakewalk and await their reply, but such an up-front bug makes me cringe.

Well, that's odd.  Your system specs look good.  Care to post up the error messages you got here on the forum?  Maybe somebody can help you out.

http://www.cakewalk.com/Support/Knowledge-Base/2016000033/Unable-to-register-the-DLL-OCX-RegSvr32-failed-with-exit-code-0x3