It's of course always a good advice to have some degree of healthy paranoia when you do not know what you are holding in your hands. I would always test for some time anyway if this is the case.
I'm using a known limestone in my tanks, still a relative hard rock containg white vains, looking like quartz but it is chalk deposites, in geologic terms still relative soft..This is a sedimentary rock formed by ocean deposites, like most limestones are and it's not uncommon to find marine life fossils in those quarrys. Sedimentary rocks are commmonly the softes because they are formed in layers as igneous rocks is formd by a melting process and commonly the hardstone category like granite.
My limestone reacts heavily to any kind of acid i put it in, i kept it in deluted muriatic acid for a week and parts of it where just eaten away. It's a kind of rock any aquarist would throw as far away as he can after seeing this..
But still I tested it for months in the water and it doesn't leach anything fast enough to make it measurable with a weekly 25% water changes. I even tested it for a month without water changes and i didn't measure any changes in ph, kh nor gh. Also plantgrowth was not noticable affected.
So imho, Cons are if you are not sure you need to test before taking risks, this takes some time.. Pros are if you do this you'll learn, e.g. that sometimes things previously thought impossible or not good aren't always as drastic as portrait.
Intresting topic actualy.. I wonder and ask myself now, why rock formed by oceanic deposites doesn't contain salt? We even have clay balls sold as pond and aquarium fert balls which are quarried from oceanic blue clay depostis. I never noticed it leaching salt, probably doesn't do with negative effect else it would be sold as fert ball for aqautic plants.
Have you ever try to find out what type of rock it was??
