I recently opened one of the UMC's up, but it's a pretty destructive process due to the housing overmold and it being glued shut. It's a "one-way" trip, so definitely do NOT try this at home!
Tesla is really pushing the limits on the UMC design! When used with a 50A adapter, It's putting out a 66.7% Duty cycle, which equates to 40A. They use paired 2.5mm wires for each side of the line, (Equivalent to ~#13AWG) and inside the box they use the same P&B relay ClipperCreek uses on the LCS-25, a single 30A rated relay. (at 40A!) Now since it's almost never going to be expected to interrupt any sizable current, this is probably fine as long as the contact system in the relay is low enough resistance and there isn't too much heat dissipated. I haven't done any thermography yet.
It's arguable that splitting the high-current between 2 smaller conductors is better for heat-dissipation, but then you sometimes have pesky termination issues. Inside the UMC, Tesla welds the 2 conductors to a little square terminal, then screws this to a PCB terminal. So this is a good termination and it looked well-done by my assessment. Having 4 smaller wires instead of 2 larger ones definitely makes for a smaller, more flexible cable overall, so regardless of any potential current handling gain, it's good ergonomically.
I definitely don't like their interchangeable right-angle plug connector design. First off, if you are going to do right-angle, why not make it symmetrical? That way you could flip it 180 degrees if your outlet is installed upside-down. (Patent-Pending! =)
Their "dongle" design makes for elegant and simple plug adapters, but it ends up being a really big blob, especially when used on smaller outlets (NEMA 5-15). This will put even more stress on the outlet, with the high bending moment due to hanging the weight of the UMC and cabling off a long adapter conflagration. I think I prefer having a short length of cable for each adapter, as this makes the final plug smaller, more compact, more flexible, and spreads out heat. This is the design choice I made on the EVSE upgrade. I use a NEMA L6-20 (L6-30 on higher-power units) as my intermediary adapter connector, which is a good design, positively locks, and still very compact. Then I use a short cable to connect the adapters to the plug, and all connections are welded and fully overmolded. This maintains flexibility, reduces physical stress on the outlets, and spreads out any heat generated. It also means you don't need an adapter at all to "go native". I decided against right-angle choices on most adapters because there is no standard for outlet installation in the wild, and you could very often end up with a 180 degree wire exit, which is not a good idea.
-Phil