As I understand it the rod bearing journals are too narrow (i.e. diameter) which reduces the physical surface area of the bearing combined with extremely tight clearances. I don't know but as d00df00d alluded to about the con-rods perhaps the rods themselves were from the BMW parts bin rather than using custom rods which would complement larger bearing journals. In any case the BMW fix was to revise the bearings with ones which have a larger clearance and a material change (tin from copper). I think the revised bearings are working fine but I stopped following when I realized that it was going to be exceedingly difficult to locate and maintain a lightly used example.
Because these engines a naturally aspirated with oversquare bores their low rpm torque is around 300 ft/lbs at almost 4,000 rpms. They really were race engines adapted for street use and I think that's what hurt them. People buy horsepower but drive torque. Almost all initial buyers were leases who used the vehicle to commute with occasional stints at the track.
I still think the E90/92 M4 is one of the best looking examples today.
Rods and pistons are shared with the S85.
It was indeed thought that the updated BMW bearings had no copper. This was later proved false, and the initial source of that info was quietly updated, though few people seem to know that. The new bearings do in fact have a trimetal construction including copper; composition is unknown, though they most likely are lead-free with an aluminum alloy top layer. It's unclear whether clearances changed; if they did, the difference is minimal. Eccentricity seems to have increased and tolerances seem to have improved significantly.
Engines with the updated BMW bearings have occasionally suffered bearing failures, though it's not clear how many, nor is it always clear why. Updated BMW bearings that are pulled from service
look discolored and worn, even after only a few thousand miles. However, it was recently demonstrated that the discoloration most commonly seen is superficial and does not represent wear through the top layer. There are multiple possible reasons for the discoloration other than wear, but no one has done the legwork to rule any of them in or out. No one's measuring these bearings, either. So, really, we have no idea how they're wearing. Better info will likely be slow to come, largely because so many people are already satisfied with the current theory about what the problem is and how to fix it.
Aftermarket bearings with increased clearance are showing considerable promise. However:
1. There haven't been enough examples pulled after significant mileage to compare to the massive number of examples of OE bearings; and
2. Although there have been engine failures with them, those failures have all been chalked up to something else (install error, other things failing first, etc.) and many are thus dismissed before they can be scrutinized or counted.