If you haven't figured out, the founding mantra of the USA is "equal opportunity, not equal outcome".
But UBI has nothing to do with equal outcome. It's about a minimum outcome, and takes the place of a myriad of support programs we already have in place to give people something to fall back on when they've got nothing else. Social Security, food stamps, unemployment, tax credits, etc -- these programs are already funded via taxes (yes, social security is more like mandated retirement planning) and would be eliminated with a UBI program. The savings in administrative overhead alone would be enormous.
encourages mediocrity and abuse
I think few people would be content with nothing more than the UBI, but if they are, then so be it. As for abuse, how can you abuse something for which everyone is eligible?
Venezuela as the latest example
This is a meaningless comparison. The Venezuelan economy has little in common with that of the US. As a single data point, in 2013 its GDP per capita was about $14,000 while the US was $53,000.
I'm not saying that a UBI is a surefire good idea, but it also shouldn't be dismissed out-of-hand as a replacement to our current welfare systems.