For many people, BI would end up being a large portion of their income, such that losing it would be catastrophic. Now those people would become beholden to any government demand made on penalty of revoking BI.
If a government enacts unconditional BI in the year 2020, who's to say they won't change their minds in 2028 and make it conditional on good behavior?
Are there any mechanisms that could make basic income a true unconditional guarantee?