I do not have empirical evidence to say exactly what happened, just a gut feeling and assumptions. Admit it or not, that's what most things come down to with mods and results.
Mine were approximately 2 years old; I forget exactly when I installed them. I have had no specific problems with the fronts, only problems at the rear. About a year ago I cracked the driver's rear upper forward at the eye while in Sedona. I believe it actually started cracking while on the Back Way to Crown King taking more difficult lines since I was traveling with other rigs and could take more chances, then it failed completely on a washboard gravel road the next morning. When that occurred I chalked it up to random luck; I had never heard of anybody having this particular failure, but then again I've broken plenty of things on this truck that most people don't, such as the front strut I snapped only a couple months prior to that.
At that time I was running 275/65-18 Goodyear Duratracs. A bit larger than the 265s most people run but not much of a difference. I run full time with Johnson Rods, and the truck sees highway use generally only to get to/from trails. It's a trail rig, not a daily driver. Extra weight wise, I have Tactical Rovers sliders, gas tank skid, and transfer case skid; a custom built rear steel bumper w/dual swing outs; ARB front bumper w/winch; Urban Offroad low-profile rack; and carry a full time load of spares and recovery gear that probably weighs around 100 lbs not including the hi-lift on the front bumper. Add to that the camping gear, water, food, fridge, and 2 people for a typical trip.
I have noticed that the truck rides a bit harsher on washboard or other sharp/harsh roads. This seems to be a direct result of the firmer bushings.
So, after that first failure I simply replaced it with what I could get my hands on while on the road which was a stock LR3 arm w/bushings. It's still in there. I kept my broken part thinking I'd switch out the poly bushings (orange, from AB FWIW) when I got around to it but….well…it just hasn't been a priority.
Last winter I switched from the Duratracs to Cooper STT Pros in 275/70-18. A little bigger at just over 33” and a little heavier (I don't recall the weight difference). Otherwise the truck is still the same.
Then about a month ago while on the Rubicon trip I had the passenger side upper forward control arm fail in exactly the same way as the driver's had previously. This seemed too much to be a coincidence. Looking at the failure in detail you can see the welds did not fail but the eye snapped off just outside the weld. Again, more supposition, but it appears that the heat from welding the eye on left the metal weaker. I also notice that the rear eye has a gusset while the forward does not.
I joke a bit about how I abuse the truck. But that's not to say I'm a lead foot who makes banzai runs at crazy obstacles; I'm not a Pin it to Win it type. I'm actually a pretty cautious driver who really does embrace the ‘slow as I can, fast as I must' ethos. I'm just willing (eager even) to push my LR3 over obstacles that most owners won't try. So while I say I abuse the truck, it's more a matter of tackling more challenges than most than crazy driving.
So why did I have identical failures on each side of the truck? The major factor I can come up with is the stiffer bushings can be felt to provide a harsher ride so it's fair to assume they also transmit more vibration and energy into the arm which results in the weakest point failing. I can't show definitive proof of this, only my best speculation and guess. I don't plan to go back to poly bushings for the parts I've broken (currently stock on the driver's upper, poly on the driver's lower, and RRS upper and lower on the passenger's side, so quite a mix for the time being). In the front I'll leave the poly in place but when they are ready to be replaced my current thinking is to go back to rubber. Not for durability but for a slightly softer ride on washboard roads.
That's about as exhaustive as I can be…not sure if it's any help but I thought I'd go ahead and lay it all out in one place.