Whenever you buy an electronic product, the possibility of it failing can be worrying. Laptops and MacBooks especially are a significant investment. Sometimes, a new product may pass every test it needs to pass to be put on the market... and somehow fails due to a manufacturing issue once it reaches its new owner.
Although random fails are always possible, bear in mind that refurbished MacBooks go through high standard quality checks twice: once when they are first put into use, and a second time as they get refurbished. Even if a defective part manages to pass the quality tests the first time around, it's very likely going to be caught, fixed or replaced during the second round of tests.
Because of this, we suspect that refurbished MacBooks are less likely to fail than new MacBooks. However, there isn't any data on this yet, so we will have to settle for saying that refurbished MacBooks are not more likely to fail than new MacBooks.