I switched from mentoring to volunteering (mostly inspecting) about the time AM14U4 and this bumper mount kit arrived, so I haven't built a robot with it, but I have inspected quite a few.

I never used the original AM14U (the team decided to do a welded frame in 2014), but I used the U2 and U3. In both cases, we were able to match drill, or at least match mark points to drill, to get the bumpers to fit. Our best tools for 2016 and 2017 were a drill press, a (wood) chop saw, and a portable band saw, and the kit chassis and bumper mounts never failed us . We may have had to drill a few holes a bit larger, but nothing more than that.

The front/rear bracket is OK, but what's with the side ears? Unless you take the chassis apart to change bumpers, they have no function. What's up with that??

The side bracket is a hot mess. A few teams use them, and use them well. About as many teams just toss them and do something completely different for side mounts. And too many teams expect them to work like a kit and are disappointed. Anything expected to mount to the KOP chassis should either be pre-measured to fit, or able to be match-drilled. And there is no way to match-drill the side mount bracket to fit a chassis. And OBTW, if the team can't cut the chassis precisely, much less place the screws precisely, the instructions don't work.

Bottom line:

The KOP chassis should work even for teams who match drill their bumpers.