Hello, @LeileifromUK
OMOP community likes challenges and has been through a lot during years 
UCUM is a valid standard for units and no one will be bothered if conversion from hours to minutes happens somewhere in your data. It is a valid strategy, but not considered best practice. Saving original data may be crucial in some cases.
I don’t see how the current system is wrong.
If you have units in your source data but don’t see them in OMOPed UCUM (you found missing ones), ask here. Otherwise, we don’t really need thousands of theoretical units of all kinds in OMOP.
Best regards, Oleg