Stephen Ambrose is no longer respected as an academic historian. The plagiarism scandal, combined with the criticism his later books have received more generally (substituting oral history for primary research, basic errors), along with the less than rigorous nature, make it not the kind of thing you would want to cite either on a college paper or here on r/askhistorians. It's fluff pop history, basically.

That said, I don't think they're absolutely worthless to read, as long as you bear in mind what you're reading. If you enjoy them (and there's quite a bit to enjoy), there's no reason not to; we all like a bit of fluff now and again. However, if you are attempting to get the best education possible on a given subject (especially WWII), there are better options.