yeah, if the pH in the reactor gets too low (<~6.3) it will mush granular media like ARM and Koralith. Schuran-style (coral skeletons) doesn't seem to have this issue and can be driven *really* hard.
if you don't keep an eye on things, your effluent drip rate can slow down as stuff builds up in the lines, which means more relative CO2 in the reactor, which means lower pH. or when you get near the end of a cylinder, regulators don't work right when the tank pressure drops below a certain threshold and can actually let alot of CO2 out while it's dying, basically a free-flow. which lowers the pH and mushes the media. mushed media can ruin pumps from all the grit it releases into the water. usually just the recirc pump on the reactor but if it's bad enough i've heard of people ruining return/circ pumps in the whole tank.
all that said though, i don't run a controller on mine mostly because it's one more probe to worry about and keep calibrated. it's always been rock solid consistent and the only times i've mushed mine is when i screwed up, and i always caught it quickly enough.
i still have a bad feeling about using a controller to manage tank pH. once you get the reactor going and keep using kalk, you'll figure out settings to get the pH good and won't need to automate pH adjustments to the tank. i'm still worried about it dumping too much kalk in, either slowly raising your sump level to the point where it overflows on pumps-off or simply spiking your pH because it doesn't think the kalk it's adding is doing anything.
