@acer
You are correct. I had the sum 5 * Unit(m) +x in mind, which does not lead to an error message in the OPs example with the error messages using Simple (but it does using Standard).
In your example, Simple temporarily assumes that x is of unit m which does not fit to the second term where x must be of unit s. No global checks accross several inputs are performed.
I never understood why Simple is called simple. It seems to be more sophisticalted than Standard (or at least allows for assignements to names/variables with and without units).
Anyway, I don't use unit packages. I am fine with subs/eval of 'name=value*unit', simplify, combine and convert after having done algebra on symbolic physical equations/expressions. Unit inconsistencies will finally reveal at the end, when numerical results are computed form the symbolic results.
Assignments to names as 'name:= value * unit' have complicated my work since they "ruin" symbolic results (which require reexecution of a worksheet when this happens). So I stopped using assignements for physical quantity and parameter definition.