I can confirm that I've used this script for many firecapes on mains, pures and zerkers. Sure sometimes it fails but it all depends on a good brew/restore balance and what settings you choose. There will always be a sweet spot that works best for it.
Also, I've done over 100 firecapes and not once got a ban, so it's def not a high ban rate.
Depending on your needs and uses for the script, I'd say $30 is not bad for a one-time payment. It takes time to program these bigger scripts and you can pretty much farm a jad pet with it or get all the firecapes you need to attempt infernal.