What problems with the S/D spindle are you referring to? I have several customers who use S/D to Analog cards to control their spindle speed and that seems to work fine.
The ones that come to mind are
Very occasionally the pulstream will stop for a split second, this also happens with the USB SS but as I have that on a mill it is hardly noticable. On the lathe however with the heavy chuck it is more noticable as you get a clunk from the backgear. As said however this is very occasional and can be fine for long periods. I only ever once managed to capture it on a scope and that was just pure luck that I was walking past at the time as it had been connected up for days without an issue.
The spindle override is poor, if you override the spindle it will be consistent for a short period of time then it will get very erratic, again this is amplified due to the weight of the chuck. I have my spindle override disabled due to this. At the start I wondered if it was the external pot that was the issue or possibly noise but I can watch the PLC and also the Brain and the numbers in it are very steady and the values in the PLC are big so any variance is easily seen.
The pulses themselves from the spindle output is not like the axes in that the width is not controlled with the velocity but rather it is fixed. This causes issues if you try to use a high steps per unit as the pulses are just too close together at higher RPMs. My servo drive can really respond to a high rate of feedback but I can not take advantage of it due to the way the spindle pulses are output. If I uses a high interpolation on the feedback encoder then I get nice control but I then have to use a large electronic gearing for the steps per unit input, this then can cause problems if stopping from higher RPMs. To confirm that it was definitely an issue with the way the spindle pulses were output I configured the spindle as an axis and the problem dissapeared.
Hood