1) Sanctuary Laws allow undocumented immigrants to live and work in such cities... for CASH. Since the employer - usually labor intensive like at Restaurants, construction sites, janitorial services etc - pays cash, the employer can pay anything, even as low as 5 to 8$ an hour. So the undocumented immigrant is getting exploited.
2) Since they are undocumented, these workers also routinely get harassed, abused by employers. I've seen 1st hand female restaurant workers touched, rubbed, groped etc in the kitchen. They can't go anywhere to complaint cos they'll get identified and deported
3) Lastly, and this is 1 of my major concerns, whenever these undocumented immigrants needs medical assistance, the city's tax payers foot the bill for the ambulance, hospitalization etc since these people (don't mean it to sound like that) also don't have health insurance. 1 Restaurant owner in the edge of the Tenderloin district told me that this population accounted for over 70K of tax payers money in 2015 (not sure how he found that out).
Lastly -- and this is not something I subscribe to, but Trump made it a big deal as it is sensational and brings in the "angry white man" voters to his fold -- undocumented immigrants have committed violent crimes in San Francisco and in 1 such case, a lady was shot point blank range while she was strolling in Fisherman's Wharf (totally unrelated to deranged shooter) and Trump made it a case of "See we allow undocumented immigrants and they come and murder us."