I don't do them for the simple reason that they are only valid in the USA and UK. I can't send out a newsletter about them, when a lot of my subscribers are in other countries. I've never had any luck with them, although I do know people who have. I do think a freebie promotion or .99 with some good promotion sites like ENT and Freebooksy work better.
You can segment your newsletter to only send to US and UK people. Even if you didn't ask people for their country. Many mailing list services will have this information.
I send out a lot of single region deals (usually US or UK but not always), but I never make them the primary reason for a newsletter.
I haven't done a countdown deal in some time, but if you are planning a limited sale, then it's a question of extra royalty vs. discounts in non US/UK regions. Most discount based newsletters are aimed at US subscribers, but some do have international subscribers, and, of course, BookBub has region specific deals.
Unless you are advertising in those other regions, a KCD makes more sense financially than manually setting the price to .99.