Obviously accuracy is important and it is true that the longer the level is, the more accurate the overall result.
How accurately the vials are put in to the body has a big impact on the overall accuracy. Special manufacturing processes and tests are used to control the accuracy to be within defined limits.
Whilst there are no international standards, C.K and Avit Aluminium levels are controlled in manufacture to be accurate to within +/- 0.5mm per metre length.
How does that compare with others? There's none with a higher level of accuracy.