Dude forgot Dunu is also Chi-Fi.

How does the Serratus compare to the U12t?

For me, the build quality of the U12t is good but not the best. I can only compare it to the OG Andromeda.

It's all metal and it feels premium but it could still be better. The face plate part where it meets the body of the U12t is not seamless, you can feel its edge a bit (like a nail file it can gather some dust) whereas the construction of the Andromeda is perfect with no gaps and excess parts.

I would not worry with those issues you've listed down with the latest U12t versions as the cable is a whole lot better than the previous one with better ear hooks. No worries on the hockey puck case, the shell would not dent unless you are not careful with them.

The 2-pin connection can be more wobbly on some models but unless it presents disconnection/channel imbalance it is not a concern and YMMV. If you are careful with them, it wouldn't be an issue unless it's a QC problem. Apparently, it is by design to protect it from wear but maybe it's just their marketing talks. The MMCX of the Andromeda is sturdy with no problems.

In what areas do they disappoint for you, small things that ended up as dealbreakers or just the overall presentation? I also still like my OG Andros.

The latest batch of U12ts include SpinFits in the box.

It's cool that they have an option for a USB-C termination w/ a built-in DAC. It's kind of expensive though as you can get a separate dongle which you can use with your other headphones/IEMs.

Off tangent, why did you replace your Trio with the Nio? Just curious.