IANA Four Wheel Drive Expert, but it seems that the key thing that hardcore off-roaders are pooh-poohing about IRS (and IFS) is the lack of heft and durability. An IS setup transmits engine power to the wheels via half shafts which are exposed and must respond to the articulation of the wheel when traveling over rough terrain. Although constant velocity joints offer an improvement over universal joints, they will still fail, especially if repeatedly subjected sudden bursts of torque in combination with severe angles–both situations common to serious off-roading. Also the boots protecting the CV joints (and containing their lubricant) are vulnerable to snags and tears a solid axle setup is basically immune to. A little bit of grit will destroy a bearing in a matter of days if not sooner.
Compare this to a solid axle in which the axle shafts are protected by the axle housing and–especially on the rear–are not subjected to flexion. Essentially the suspension flexes so that the axle shafts don’t have to; no matter how the body or suspension of a truck is twisted, without CV-joints or U-joints to bind, solid axles will spin as freely as if they were traveling down the highway. This relationship transfers the pressure to the joints of the rear drive shaft which, I believe, are the most likely suspects for failure in an off road enviroment (in the rear end). Still this is an improvement, as a main drive shaft is substantially more robust than a half shaft.
More factors that elevate SA setups over IS setups (in built 4x4’s) are cheapness, availability of parts, and ease of installation. Many beefy axles are biding their time in junk yards, just begging for some overzealous gear head to come by and swap them under their rig. Suspension geometry is complicated, but much more so in IS setups. Your average shade tree mechanic probably couldn’t reconstruct (or modifiy) an IS without extensive training, but probably could come up with an axle/spring/shock absorber setup that was workable–especially if the rig is meant mostly for beating around trails.
Basically for a little jaunt in the woods–during which the average driver would probably be too timid to push any 4x4 to the point of mechanical failure–the cute-utes would fair well enough (taking into account their decreased ground clearance, less ‘torquey’ engines and smaller tire sizes.) OTOH, someone who is going to spend the summer testing and redesigning their roll cage will want strength, reliability and simplicity–which means solid axles.
I forgot to mention aftermarket tires and will do so quickly; suffice it to say a 36" tire demands more torque than a stock tire, further stressing whatever sort of shaft is driving it. Dammit-also forgot locking differentials (in SA), which effectively lock the wheels on an axle together and are one of the first upgrades a serious off-roader is likely to make.
I’m sure Rickjay will happen along any moment to blow my post apart like a welded differential–but I hope he has time to point out the efficiencies gained via strictly mechanical (metal on metal) transmission of torque versus the viscous couplings common in the center differential of cute-utes.
BTW, Belrix, The H1 has geared hubs, meaning the half shafts don’t power the wheel directly, but do so via a gear at each wheel, which allows the half shafts to be further from the ground and operate at less severe angles; I don’t think this is standard on RAV-4’s.