Very few unemployed people would be incapable of doing work with at least some value for a prospective employer. In order for someone to be worth hiring, however, it is necessary that the value of their work to the employer be sufficient that the employer can in turn pay the worker an amount sufficient to justify the worker's efforts. Having people who could be doing work with some value refrain from doing so is harmful in many ways. Any policy makers who are genuinely benevolent should endeavor to minimize such unemployment, and should be careful to ensure that none of their policies instead promote it. Conversely, policy makers who are interested in political power may benefit by implementing policies which raise the minimum level of labor value necessary to make someone employable, and then promoting policies which "support" the unemployed but in fact make them even less employable.
Depending upon where the money comes from, a "basic guaranteed income" may increase the minimum level of labor value necessary to make an employer/employee relationship worthwhile for both parties. Such policies, however, could be a good thing if they can replace other social welfare policies which are much worse. Many social welfare policies are constructed so that people on welfare who accept low-income jobs have their welfare benefits reduced substantially, leaving them little reward for their labor. Thus, unless a sense of pride from reducing one's welfare dependency can serve as a reward in and of itself, such policies serve to greatly increase the minimum labor value that would make it worthwhile for someone to work. Replacing such policies with a "basic guaranteed income" would greatly reduce those effects, and encourage such people to work. Employment of the presently-unemployable could be enhanced further if the "basic guaranteed income" made it possible to eliminate the minimum wage (since the minimum "livable wage" would be $0).
With regard to a "Basic Income" which doesn't replace worse aspects of a country's social welfare system but is instead takes the form of resources injected from outside a country, the effects may be harder to predict. An externally-supplied "income" which is too large relative to the value of a person's labor could discourage people from working. Further, there's no way to avoid having an outside injection of resources affect the prices of things. An injection of useful resources won't cause massive inflation the same way an injection of paper money would, but the effects on market prices may nonetheless be harmful. For example, if the primary use of labor in a region would be to harvest grain, then an outside injection of some quantity of grain per person might ensure that nobody starves, but could on the flip side greatly reduce the value of farm workers' labor. Freeing up the labor of people who would otherwise work on farms would mean that other industries or prospective industries would have a new source of labor available; that could be a good thing if capital exists to create industries to utilize such labor, but would not be a good thing if no new industry is formed and people are left without any useful job to do. Harm from poorly- or malevolently-designed social programs need not result only from the fact that any resources they distribute need to come from somewhere. Even if a program only injects resources which were not taken from anyone with whom the recipients might interact directly or indirectly, it's possible that such injection may undermine the society receiving it.