I have now 22 GB RAM and I'm working only in SSD drives. This are my impressions:
1st. The EuCon bug is still here, but sometimes Nuendo does not crash in Mountain Lion but it always does it in Mavericks. (WTF)
2nd. The system seems more reliable. Almost no crashes.
3rd. Video service problem is even worse. Now I have CRASHES in the video engine module (see the activity monitor). I found that all my CPU peaks are related to it.
And now, the good news. How I solved the the video service issue
Copy the videoengine.bundle from inside your 5.5.x installation (Nuendo 5.5.app/Contents/Components) to your N 6 components folder (you should rename the older one to something else to avoid overwriting).
Everything is now stable, no issues, no CPU peaks. The bad side of the fix is that you're going to lose the video thumbnail cache, but it's a small problem for a great benefit.
Nuendo 6.0.x / Cubase 7.5.x
Mac Pro 3,1 Octo 2.8 Mhz, 8GB RAM, OS X 10.8.4
RME HDSPe, UAD2 Quad, 6.5.2, Novation Nocturne (as MIDI device), Unitor 8 (x2), ESI Pro MIDI, Blackmagic Intensity Pro
MacBook Air 4,2 4GB RAM, OS X 10.9.2