The app itself was deficient in testing, had security issues, wrongly told thousands of people to quarantine, was not as accessible as it should have been, and was overall very mediocre software. If that's what my federal government can deliver (albeit through contractors and middlemen all the way down), I don't want them anywhere near my finances.
Public adoption was mandatory for a time, so that aspect couldn't have failed.
And of course, the Canada Revenue Agency is clearly able to manage people's taxes and aspects of finances safely and securely, but that has decades of effort and tremendous financial investment behind it. A greenfield effort seems a lot less safe, and I trust the public service much less to execute on that coherently, consistently, and effectively enough to deliver something better than we presently have.