a. Obviously windshield solution cannot be used in potable systems; methanol is toxic (very) and the Bitrex will have you spitting all summer.
b. The alcohol can evaporate. In a car you keep using it and the system is sealed, but the head, for example, is quite open.
c. Some mixes are flammable.
d. Methanol is hard on certain plastics. The system in your car is designed to methanol, your boat is not. Both PG and methanol craze clear strainers.
e. Corrosion. Quality PG winterizing fluids have effective anti-corrosion packages, windshield cleaners do not. I did corrosion coupon testing and the difference was sobering.
f. Even ethanol based RV winterizing fluids suck. Practical sailor just did a review, explaining the details based upon testing.
g. Yes, glycol fluids slush below zero, but that does not related to bursting. Take some, put it in your deep freeze in a glass jar, and try to get it to burst. You'll be surprised how difficult it is. Alcohol fluids transition to bursting more sharply... particularly after the alcohol evaporates while waiting from October into February.
You could use it, but most consider this penny wise and pound foolish. Save $4 and damage equipment. Ick. Buy quality fluids, make CERTAIN that it does not contain alcohol (even good brands often make a quality fluid and a price-point fluid using ethanol), and make certain that the material coming out the other end is fully treated (check the freeze point).
