Yeah IRS is one of those things people latch on to. What they dont say is that a properly setup solid axle will handle better than a poorly setup IRS. Now I know a proper IRS will handle better just saying that a solid axle can be made to work.
it's real easy to have poor alignment on IRS. i've chewed up 2 sets of tires and spun out due to it being out. There is camber that's pretty easy but the rear toe is a PITA.