Kimbers have a plastic mainspring housing, again not on current production models I have seen.
My current UCII has a plastic MSH
The only failures I have had with my UCII, have been junk Kimber mag related. Wilson mags have solved my premature slide lock issue.
The only other issue has been a grip safety timing issue with the firing pin block. You can push the safety in far enough to get the hammer to fall, but not enough to clear the firing pin safety, resulting in a click no bang. Easy fixes to both issues are to eliminate the junk factory mag, and remove moving parts from under the rear sight
I trust my live with my Kimber, since I fixed the factory QC issues myself.