I feel your pain as I have experienced it often enough myself. Every now and then I have a go at mastering DCs and am in process of doing so now with time on my hands.
As @john_mcclenahan has pointed out, DCs have never really evolved since their first outing when announced to great fanfare. Lots of quirky things about them would probably have been ironed out by now if developers had retained an interest in them. If you have watched any of @TheOnlyAaron’s 8 part tutorial on them, you will see that even he, in a prepared for video, keeps forgetting to press enter after entering a formula. And so so I. It’s very frustrating.
Like you (and most of the planet), I work in metric but you probably have to make some allowances for something developed in the US. You do get used to it, but do occasionally get tripped up. Sometimes, without you realizing it, the little ruler slyly reverts to inches. You can spend some time wondering why things aren’t working the way you expect until you clock that you need to revert to cm.
Another headache is the fact that it uses cm and cannot accept mm, the unit that architects (and engineers?) use most. If you are inputting user defined optional sizes into Component attributes, you have to keep remembering that what you think of as 500mm will be interpreted by the system as half a metre and so you enter 500mm in one box and 50cm in the other. Not too hard but have to wonder why input is made so hard.
DCs can be mastered as there are some people who have managed spectacular things with them. But the learning curve is steep.