Here's my theory. It's long been known that the RDSport sway bars don't fit properly. UUC tried to design a bar that fits properly, but, IMO, failed miserably. I had the UUC sways on my car for a while. When installing the rear bar, the actual mounting tab is way off the perpendicular, vertical plane with the spring perch (or whatever the suspension piece that the sway bar attaches to is called). So the heim joint links that UUC provides must be installed at a severe angle (if I recall, something like 30 degrees off perpendicular) in order to connect the sway bar with the suspension piece.
When installing the UUC bars, I actually ground off a piece of the heim joint for fear of binding issues.
So...when you have the links at an angle, when the sway bar pushes up on the link, not only is there a vertical force, but a horizontal one as well. This leads to two things: a) accelerated wear of the heim joints (my friend had his UUC links w/ UUC bar links fail with under 10K miles....remember, heim joints are designed to work with radial loads) and b) lateral movement of the sway bar.
The bend in the bar next to the support bushing is soooo close to the bushing/tab area, that any bit of lateral movement will place side pressure (and I would think perhaps more than 100 lbs of force) on that swaybar tab. What's the result? Failure of the tab.
My friend had both of his rear swaybar tabs fail (his car is a '99M3...so it has the reinforcement that some people speak of)...and they started tearing from the outside in...like it was peeling the metal off. The bar he has...UUC. I've never seen the RDSport bars, but if they don't fit as well, I'd venture to say that if people are experiencing failures with those bars, then it is probably for similar reasons.
Since I didn't like the fit of the UUC bars and didn't feel they were engineered correctly (and the advertising of 26/24 was misleading), I swapped for Eibachs. Eibach bars are an *exact* fit. They reuse a modified factory rear sway bar link (bushing pressed in to accommodate a big bolt) and when all is said and done, the rear sway links stand nice and vertical. So when the sway bar pushes up, the force goes to twisting the sway bar (what they were designed to do) and not to push the sway bar sideways.
My rear sway bar tabs are fine ('99M3) as well as another friend who drives much harder than me ('99M3 as well...and he actually made the factory front sway bar links fail...you can tell by play at the ball joint under the rubber boot) w/ Eibachs.
Now how many people with Eibachs out there have exhibited the rear sway bar tab failure? I'd like to know...