9v batteries don't carry much energy. Either this light will not be very bright, or it'll have a relatively short runtime. Or both. And 9v batteries are kind of pricey to replace.

As I see it, for taillights it's hard to beat a number of these, two or four or whatever your budget and space permits. You could get a flashlight that's brighter, of course, but as long as the car can see you within a block or two it's all good, and some of the flashlights do have a fairly tightly focused beam which would make it difficult to see unless one is directly behind you. Get two or more of these at various angles and nobody's going to miss you from anything even close to behind you.