Bought similar recently to cut the mortar out of a lead flashing - so it could be re-used. Worked ok but I doubt it would be very effective on anything much harder or more substantial. On the other hand I also bought some masonry blades for a sabre saw and they worked brilliantly. Well, much, much better than I was expecting anyway.
you have perhaps several 1000th the wear rate on a grinder over an oscillating tool as it goes to and fro perhaps 6-8mm on a very small point on the blade rather than several thousand feet per minuite as pi r squared [125mm x3.2 times rpm]