Is HIIT good for cardiovascular health?

Yes — the evidence for this is strong. High-intensity interval training (HIIT) is supported by several meta-analyses and randomized controlled trials showing improvements in cardiorespiratory fitness and body composition, including in clinical populations such as people with rheumatoid arthritis, stroke survivors, and adolescents with obesity. That's reflected in an evidence score of 75/100 for its role in cardiovascular health, drawn from 6 studies (4 RCTs), with an estimated impact on longevity rated moderate (65/100).