There’s no such thing as bug-free software. Through a QA process, teams just try to fix or work around the most egregious ones. To fix them all would actually be a bad idea - no software would ever be finished. And, yes, it has much to do with the complexity of more modern pins, not only in terms of the gameplay logic, but in all the disparate technology (hardware + software) that needs to be wrangled to work together.