Sorry… I worded it very badly… Of course every software has various issues… I just meant if that was the main showstopper CAUSED specifically by mac os versions higher than 10.13.3.
For example, pro tools has problems with massive cpu spikes if the buffer is lower than 128, or 256 at higher sample rates. Pro Tools performance is bad at low buffers anyway, but this just makes it unusable, and it only happens when you upgrade to High Sierra 10.13.4, .5 or .6…
Is Cubase suffering from similar low buffer performance issues?
Right now, on OS 12.12.6, Cubase 9.5.30 has never run better for me… Performance is edging out Logic 10.4.1 by around 10% VI polyphony and when tracks are armed, Cubase can edge Logic out by as much as almost 50% from 128 buffer and upwards. Logic wins at 32 buffer… but the reason this happens is that Cubase uses all available threads when tracks are armed for record monitoring, and Logic will only assign half of the system’s available threads… So… at 64 buffer, Cubase just wins, but at 128, it thrashes Logic by even almost double simply because it is using all 8 threads on the system (for example when using a quad core i7 cpu) whereas Logic will only multi thread live tracks across 4. This applies to armed VI tracks, anything that is in record arm/monitor arm mode.
I am scared about High Sierra, but my computers are so old so the imac Pro is my most significant investment to my studio in years and I am also very excited about it… But I’d really like to work at 64 buffer if possible. I work at 128 now and one of the main reasons to upgrade my machine IS to be able to work at 64 buffer to play VI’s, particularly pianos and drums where I need that “instant” feel, as well as some guitar playing through native guitar amps.
A very kind member here provided me with the full installer of OS 10.13.3, so as soon as i receive the machine, I can format it and install the older OS, as I don’t need the features of 10.3.4 anyway (the main one is egpu).
So I guess I am asking the question, if 10.13.3 performs more stable at lower buffers, like pro tools, then I will happily install the older OS. Unfortunately sierra can not be used on imac Pro’s as there is no embedded hardware support.
If you guys believe, that besides the mixdown freeze bug, that cubase 9.5.3 otherwise performs just as well on newer High Sierra than the older pre 10.13.4 versions, then I won’t bother mucking about with the machine and will leave it exactly as is.
As you can see here, Reason also suffered from 10.13.4
https://help.propellerheads.se/hc/en-us/articles/360003035494-Reason-performance-issues-with-macOS-High-Sierra-10-13-4
AGAIN with performance issues… it seems that for whatever DAW’s exhibited problems with the newer OS, they were all performance related unfortunately.
I just want to get the very best out of Cubase I can get with my machine, this is also why I ordered before Mojave comes out as that is going to break so many things. You guys already using High Sierra know best which is why I ask… If it’s preferred i open a ticket to ask steinberg which is the best version of the OS to use for performance at low buffer, I am happy to do that too.
Cheers and thanks for reading, I know i can be long winded… 
But I would say it’s the only one known issue.