Starbucks evaluates customer service at its cafes across the country using connection scores, which are crowd-sourced ratings.
The system, which has not previously been reported on, made 17 current and five former Starbucks staff feel powerless and at the mercy of customers' whims, according to 17 current and five former employees.
Workers said that low results drove management to cut hours for individual retail employees in two cases.
While the ratings have been in place for years, many workers claim that the connection score system has aided in the current Starbucks national labor organizing campaign.
Employees at more than 80 of the company's roughly 9,000 company-run U.S. locations have voted to unionize since December, a move lauded by the White House and credited for reviving other labor campaigns.
"It's just a pretty good indication of how the partners feel about corporate being out of touch with the reality of the job," Maddie Vanhook, a Starbucks café employee, said.