So, you are thinking that round(digits = 3.12) should return an error? The only required argument for the function is the number to round, which would give you the default 0 digits. Or are you worried about this possibility?
round(digits = 3.12, 1)
#> [1] 1
The x object is not preserved after the command, so the name never really comes into play. You can, of course, do the following if you want to freak out people reading your code:
digits = 3.12
round(digits)
#> [1] 3
round(digits, digits = 1)
#> [1] 3.1
In general, R will let you name things almost what you want (not that you should for your own sanity sake however), so I do not see anything inherently wrong with being able to call an object "digits".