Sobriety tests are notorious for being very subjective and not having well defined criteria and cops failing people even if they aren't drunk. The subjectivity in the test is a feature that allows cops to justify their arrests or uses of force.

Obviously once you hit a certain point of drunkness obviously maybe a test like walking in a line can demonstrate something useful. But so would a breathalyzer. The false positives is the problem because they're being used to illegitimately subject innocent people to criminal charges.