We haven't found any published statements against Measure S, but based on similar policies in other states, critics have argued that allowing additional public bidding criteria could make the process more subjective, leading to nepotism and corruption. While the current public bidding process can accrue additional costs overtime and extend timelines, it can also be argued that it's concrete enough to prevent government officials from modifying the criteria to give their buddies contracts.
It’s a balancing act though. If the city requires too many projects to be publicly bid on, then public works projects become painfully slow and expensive; if it restricts the number of projects, then businesses with established government connections may have an advantage to win contracts, reducing competition.