If my ukemi depends on someone else in any way beyond the initial taking my balance, I am doing it wrong. My ukemi is for keeping me safe, no matter who is throwing me and how......
Has your partner ever let go of all support while you were at 1/3 of a breakfall cycle? Or even worse pulled all the weight down while you were "writing" a lovely circle in the air?
I have witnessed and experienced both (along with a lot more colorful situations) and the end result was always injury, no matter how advanced the uke was.
So... i only take breakfalls from people i know and trust.