Yes, impactful is a word, but it’s likely to annoy at least some of your readers. If you want to substitute another word for impactful, use a synonym such as influential, powerful, or effective. Are all the words in the dictionary equal? Some people feel that certain words are better...
the fact thatimpactfulhas become so entrenched in the language suggests that many people don’t find it to be an exact synonym of those words and that it has shades of meaning all its own.