As it happens I've been pondering this myself lately, and by fortunate chance I got the opportunity to speak with the LFS owner for the first time last night. This was my topic of choice.
The takeaway for me was Kalk is likely advantageous in pricing, as well as its ability to combine the effects usually found by dosing multiple forms of chemical supplementation.
Kalkwasser dosing is a process that has been used for a long time within the reefing community, and when you read the notes of experienced writers and aquarium veterans (e.g. Julian Spring), many are a proponent of this approach.
The downside is the need for careful setup and monitoring when you do the process, especially when you are first learning to do so. I feel confident over multiple attempts and years of using this method it becomes extremely effective and safe, however, for someone like myself who will attempt it for the first time you need to make sure you're well informed and that you setup the process correctly. Much of this challenge can be eliminated with newer methods of dosing.
I hope that helps,
Para