To Rusty's comment on how long an IRS suspension will hold up, we run a stock F-150 front IFS differential and axles with a locker on it in our 500+ cubic inch, 1000 hp Ford mud race truck. Haven't had an issue yet. Conversely, on the Chevy IRS at about 750 hp with an open differential, we've broken many axles and a case or two. We switched to RCV axles on it and solved the axle problem but not the the case problem. Chevy IFS front ends are junk. Basically built to fail. Anyone that knows someone with a Chevy IFS, has probably seen it running around at some point with no axle on one side or the other. That's why, this off-season, we are putting a Ford front end under the Chevy asking with a 9 inch rear. While I realize I'm comparing IFS to IRS, if the Ford IFS can stand up to 1000hp while locked, turning and using all of its suspension travel (and then some) I think the engineering is available to design an IRS that can handle the job.
Technically, I see no need for an IRS for most uses, but that doesn't mean it can't be done reliably. Yes, there are more moving parts, but properly maintained, that shouldn't be much of an issue.
BTW, there were some pretty die hard solid axle folks in our sport that mocked our IFS trucks to start with. That changed pretty quickly when we started wearing their a$$ out with less hp. Most have switched to IFS now.