While sanctuary cities have legal issues, sanctuary-state status for California takes matters to a different level, IMHO, but then again I'm not a lawyer. For example,
The majority of Californians dislike the current state of immigration law, but forcing us to choose between the U.S. or the California Constitution may provide an unpleasant surprise to those who pushed this dislike too far.
The law enforcement oath of office swears allegiance to "the Constitution of the United States and the Constitution of the State of California." When State and Federal law conflict, what is a poor LEO to do? In certain situations hesitation kills. The Supremacy Clause of the U.S. Constitution subordinates State law to Federal law. The State of California may say Article VI of the U.S. Constitution does not apply, but some U.S. citizens living in California may disagree. If such dissenters report undocumented immigrants to ICE, will California prosecute them? "the California Constitution strongly protects the corporate existence of cities and counties and grants them broad plenary home rule powers. The Constitution gives charter cities, in particular, supreme authority over municipal affairs, even allowing such cities' local laws to trump state law" [Wikipedia]. California has 121 charter cities, not all of whom will agree with California being a sanctuary state. What will be the State's response if a charter city cooperates with ICE? The U.S. government has many weapons at its disposal to force compliance, ranging from withholding Federal payments to refusing to grant any California green cards to shutting off key information (e.g., successful IRS audits of Californians) that the State needs to perform its business.
[Update - 4/9: Law enforcement officers for counties in non-coastal California will support Federal law. If California becomes a sanctuary state, this county will resist]