I would ''presume'': ***increased signal to noise ratio*** (bad), and a ***significant lack of over current protection*** (which could damage input gates) These devices are amplifying tiny signals, so they don't have much protection. 10 ohm in the leads at 1v is a max of 100mA, Microcontrollers (which are not ECG machines) don't like to see more than circa 60mA at sensor inputs.
Replacing these leads with the correct type will run a cost around $50-$100. If you are using these in any type of health capacity, and you have a liability insurance policy in place, you might want to check if using incorrect leads will alter your insurance coverage.