The financial aspects of UBI are not the only consideration. Stable societies depend on strong social links between their citizens, and UBI could affect those links in meaningful ways.
Income is far from the only meaningful aspect of employment. Peoples' work and their associated skills are deeply linked to their social status, sense of self-worth, friendships and social networks. By removing the need to work and incentivizing people to stay at home, UBI undermines the social fabric on a fundamental level. The deleterious effects of UBI would resemble those seen in areas where unemployment is prevalent. In places where fewer people work, crime and drug addiction are higher, and family structures are more likely to break down. UBI would increase these social problems. Furthermore, UBI would decrease the incentive to work in areas that are already ridden by social problems, making them even more disconnected from the rest of society and offering no increase in inclusion into a greater community.
There is little evidence that people stop working once their basic needs are met. In fact, almost all societies show overwhelming evidence in the other direction. The assertion that "income is far from the only meaningful aspect of employment" is a reason why people are likely to continue working with UBI, rather than stop working. 
Social cohesion depends on core values; like the idea that wealth is connected to work.
[P1] UBI removes the need to work, which will result in fewer people working. [P2] When people do not work, socially destructive outcomes occur.
Rejecting the premises
[Rejecting P1] UBI does not provide all the benefits of employment, so people still need to work to supplement their income. [ P2] When people do not work because they don't have to, they have more time to focus on how to get ahead in life rather than developing dysfunctional behaviors.