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.