I used the BSD version of this (which seems the same AFAICT) a few times years ago and managed to misunderstand the output a few times and gave up. I doubt it was an actual error in the conversion but the UI is comically bad. The typical search engine UI is great, ask for a conversion using whatever abbreviations you want and it gives you a detailed explanation of what it thinks you asked for and the result (and not some other number you didn't ask for). I see GNU units at least has a -1 option to just give the result.
Firefox has a hidden simple unit conversion tool that can be enabled by setting browser.urlbar.unitConversion.enabled to true. It only has a few conversions, though, and doesn't give much detail about what it thinks you asked for.
It looks like there is a rust unit conversion utility called rink that seems better although, like many rust utilities, it lacks a man page or any local documentation besides a brief --help message (there is an online manual). Some units are a bit annoying; you can't just do c to f, only degC to degF (and not even degc to degf either). But much nicer than units and can also be easily be used as an interactive calculator.
https://rinkcalc.app/
Edit: Playing around with rink a bit more it has some neat stuff. It gives info if you enter a unit (or constant or substance), usually defined in terms of the base unit of each type but if you give the base unit it has a brief text explaining how the base unit is defined. "c to f" not working is because it doesn't use context and c is the speed of light. You can use 'units for' to see available units for a particular type, although it doesn't look like you can show all available units other than checking the source (based on the GNU units definitions but with some differences, such as degC). It suggests a supported unit if you type one it doesn't understand.