I haven’t had any problem with spindle control using UGS.
The only issue I ran into is related to setting the grbl configuration to use the speed controller. There is a minimum speed setting below which grbl will not allow a lower speed. On the top side of the speed range, a command to go higher than the maximum speed setting will cause the spindle to stop. Example: my spindle maximum speed rating is 12,000rpm. If the “S” command is set to S12000, no problem; attempting to set a higher speed causes the spindle not to operate; i.e., sending a command of S13000, then an M3 will not start the spindle. So, I have to be sure that any CAM software I use doesn’t set a speed higher than 12000.