I've done kalkwasser, 2-part, and All for Reef as well. Anything "liquidly" is a little bit too messy for me or I'm lazy? lol
Kalk, 2-part, and All 4 Reef are awesome for pH boost but the time & maintenance commitment is higher compared to a CaRx. For the CaRx, I don't have to replace the media once a year (perhaps longer now with my bigger GEO). I was mixing kalk/2-part/All 4 Reef stuff every week/or two with a fully grown SPS tank!
Magnesium: the CaRx does not dissolve Mg back to the water column & I didn't put the Mg media this round -- will do next time. I manually dose Mg twice a year in the meantime.
Trace elements: I don't think the CaRx will provide enough (or if any) trace elements for the reef aquarium. Granted that I have not done a WC close to 3 years and have not done an ICP test, things are growing and I have decent color, why bother? When things deteriorate, I will look into this.