most car tweeters are 6 ohms anyway, so should be fine.
the main difference with most car tweeters is they are designed to work well off-axis, i.e. not pointing straight at you. home tweeters are usually designed to point right at you, and may not work best in a car environment. sometimes also the materials are not designed for the hot/humid car and may fail prematurely. They are a lot cheaper though.