Major pitfalls of this approach? What am I really missing by not using a HR monitor? Is the greater danger going too consistently hard, or not hard enough? Recovery monitoring?
You have been training for thirty years, so most likely you have a good sense of your training intensity zones and what it feels like.
For many people who are new to training, estimating training zones is tough and using generic formulas is not accurate. Using a heart rate monitor to do AeT/AnT tests and establish heart rate zones would be recommended. Also, on aerobic workouts, monitoring the heart helps to stay in the desired zone.
The other benefit for people using a structured training plan in Training Peaks is to track their metrics – CTL, TSS, etc. Although an estimate, these metrics help to see the progression over time and plan for an event.
You might want to read the Uphill Athlete series on Monitoring Intensity.