Addison's disease involves insufficient adrenal hormone production, causing a potentially fatal crisis but allowing good quality of life with appropriate lifelong management.
Addison's disease welfare involves the acute crisis followed by lifelong management burden. The Addisonian crisis is a terrifying welfare emergency: the dog collapses, vomits, may have cardiac arrhythmias from hyperkalemia, and without immediate treatment, dies. Dogs that survive the crisis begin a management regime that requires owner commitment to monthly injections and careful stress management. Under-dosing during stressful events causes partial crises; over-dosing causes Cushingoid side effects. The welfare outcome with excellent management is genuinely good - many Addisonian dogs live comfortable, active lives for years after diagnosis.