I know it can be done without pain to the partner. I know I'm only trying to take their balance. I just don't know how to hold it without some discomfort on uke's part, not without losing control of uke anyway.
Discomfort occurs at Uke's wrist. Not shoulders or elbows etc.
At some point, uke has to protect themselves with their ukemi. They have to absorb the control in their body, move to a place in a way to take the pressure off, or tap (or some combination of these). It's uke's responsibility to let shite know if there is too much pain or tortion.
Of course, this is assuming a reasonable and responsible shite.