Rhubarbs contain nearly twice as much oxalates than spinach, excess oxalates cause kidney stones.
I like to share my experience during the kidney donor eval phase. My wife the donor during the evaluation was found to have kidney stones and at one point I felt i was very near to the transplant phase ...but still very far from reality ! I must thank the Kidney donor Dr who identified the issue and told my wife to stop taking calcium supplements which worked but it took 2 months but it came through... you can understand what was going on in my mind for those 2 months.
For anyone has been identified with CKD... they should try avoid rhubarb & spinach. Also both spinach & rhubarb contain a lot of vitamin A which helps to clot the blood... so in my view all CKD people should exercise restrain.