YSK because the first hot spell is when most people discover their AC unit is broken.
Several years ago I waited to turn on my AC. This is how I found out about one of the busiest times for repairmen and had to sweat for two weeks. Now I do a test run a couple of weeks before it heats up. Same goes for the furnace at the end of Autumn.
Edit to add: YSK line (Rule 2)
Having a boiler + heat pump would be a Hybrid system. A 2-stage heat pump typically refers to the capability for the heat pump to run at lower power (such as 70% max) or full power, to try and prevent thrashing on slightly warm days, while keeping capacity available for hot days.