Narrow crown, yes. Use galvanized 1-1/2″ staples. So long as the pickets aren’t more than 1/2-inch thick, they hold pretty well.

Stapler gun for cedar pickets?

You’ll find it hard to beat a cordless narrow crown stapler:
https://www.protoolreviews.com/tools/power/cordless/nail-guns/ryobi-p360-narrow-crown-stapler-review/9762/
https://www.protoolreviews.com/tools/power/cordless/nail-guns/milwaukee-m18-fuel-narrow-crown-stapler/48444/
https://www.protoolreviews.com/trades/carpentry/dewalt-20v-max-18-gauge-stapler-announced/31222/

Alternate your driving angles for a better hold.

And you have the Stockage fencing stapler for driving something larger than narrow crown.
https://www.protoolreviews.com/trades/landscaping/stockade-st400i-cordless-fence-post-stapler/24904/

So if this tool is like DeWalt’s already established trim nailer(s), we may purchase this for small roof/water leakage repairs we perform at times for our customers.

That being said, this looks like a good tool that should do what it is designed to do. However, we all know that this will be picked up by a lot of guys, who think it is supposed to be a full roof job tool, and use it in such a manner. Then half way through the job, they’ll see it is slower than air, and think it is junk and either return it, or throw it off the roof and be frustrated and disgusted. Then we’ll hear all sorts of stories online about how it is junk and doesn’t work etc.

For DeWalt’s sake, I hope that number is actually low, and a majority of the early adopters learn what it is supposed to be used for and use it properly, thus reporting to the rest of the world how it is a great tool, and they can’t wait for the next iteration to show up on the shelves.