Software development is not a static process and there are quite a lot of variables in an application that is data driven as accounting software. That's not to say that bugs don't happen and problems don't exist. We continue to test against Yosemite pre-release updates as Apple makes them available to developers before the general public receives them. We tested against Yosemite GM after it went public. We tested against Yosemite pre-releases before Apple went public.
The QBMac development team spends quite a lot of man years every year testing our releases against Mac OS releases. I'm sorry to hear that you're encountering problems using QBMac 2015.
