User-defined literals are basically normal function calls with a fancy syntax. [...] While user defined literals look very neat, they are not much more than syntactic sugar. There is not much difference between defining and calling a literal operator with "foo"_bar and doing the same with an ordinary function as bar("foo"). In theory, we could write literal operators that have side effects and do anything we want, like a normal function.
Obviously the arbitrary-function-part of that will never happen in Python (yes?)
Also, for discussion, remember to make the distinction between 'units' (amps, meters, seconds) and 'prefixes' (micro, milli, kilo, mega). Right away from comments, it seems 1_m could look like 1 meter to some, or 0.001 to others. Typically when I need to enter very small/large literals, I'll use "engineering" SI notation (powers divisible by 3 that correspond to the prefixes): 0.1e-9 = 0.1 micro____.