I think highest/easiest ROI would be devices that alert or avoid bad events.
- freezer/fridge temp monitors to avoid food going bad (circuit failure, door cracked open, part failure)
- water leak detectors (water heater, bathroom, sink)
- open door alarms ( garage door left open overnight, fence door left open that lets dog out)
- doorbell camera/microphone (unwanted visitors)
A lot of smart outlets develop problems over time from the inductive surge current on electric motor startup (air conditioner, fridge, dehumidifier). The current ratings in ads are generally for resistive loads. Inductive load limits can be 30% less.