Good subject to bring up.
ABC extinguishers, most typically dry chemical, work on anything by smothering it. But yeah it is a corrosive powder (Monoammonium phosphate) about as fine as talcum powder, and it gets everywhere. You spray it on your car, the 'corollary damage' can be as great or greater than the fire damage.
CO2 extinguishers work on liquids and electrical- once the power is turned off- but not so well on solids because it is a gas which dissipates quickly. Liquid or electrical won't flare up like a solid will when the gas dissipates.
A disadvantage of CO2 is that the spray can conduct electricity. When the high pressure gas in the cylinder at ambient temperature is released, the temperature drops and the moisture in the air it comes in contact with condenses. That is what the fog is, not the gas itself. But this moist air can conduct an electric charge back to the container & user. That is why they have a plastic insulating nozzle on them. (You do not want to spray CO2 onto a live electric panel!) But with CO2 you spray it, put out the fire, and get back to work.
A CO2 bottle small enough to fit in a car would have so little gas in it it would be all but useless, especially in the open atmosphere like the side of the road.
Dry powder extinguishers of the typical small size, 12" tall or so, do have only about 7-10 seconds worth of chemical in them. The shorter ones, 8 or 9" tall like you see in a lot of cars, have probably only 5 seconds worth. That's not much when you're in a near panic situation with your car on fire.
I carry two dry chemical ones in my car, a smaller 9" one clipped to the t-top harness clip bracket between the seats for immediate access and a 12" one in the back.
I like that Halotron one, I hadn't heard of it before. If it is effective in open atmosphere, it would be worth $120 to have one for my car, but I'd still carry a cheap dry chemical one to use on somebody else's car.....(Knock on wood, I've only had to put out fires on other peoples cars & bikes. I got tired of destroying my jackets on carb fires so I took to carrying an extinguisher.)
I will look into those Halotron ones. I see they have a 2 1/2 lb that would work in a car which has a 9 second spray time, and I like that fact that it comes out liquid and then evaporates. I wonder why I have never heard of it before? I will talk with the Fire Marshall across the hall and then go visit Valley Fire Extinguisher this week too.
And yeah, in my garage, I have several dry chemical & one 20 lb CO2 extinguishers. And a fire blanket as well.