Nearly every household object now has a connected version. A few are genuinely useful. Most add complexity, a subscription, and a device that stops working when a company loses interest.
What is worth it
Heating control. A thermostat that learns your schedule and can be adjusted remotely pays for itself in most climates and is the clearest case.
Lighting, specifically for lights that are awkward to reach or that you want on a schedule. Not for every bulb in the house.
Sensors – water leak, smoke, door. Cheap, passive, and useful precisely because they notice something you are not there to notice.
Robot vacuums, which are the most consistently used smart device in most homes that have one.
What is usually not
Anything where the connected version replaces a mechanical action that already worked. Connected kettles, fridges with screens, taps.
Anything requiring a subscription for basic function, particularly cameras that store footage only in the cloud.
Devices from small companies without a track record, since the service disappearing turns the device into a paperweight.
The questions to ask before buying
Does it work if the internet is down. Does it need a subscription. Does it work with the systems you already have, or does it need its own app. What happens when the manufacturer stops supporting it.
The security part
Every connected device is a computer on your network, usually with poor update practices. Put them on a separate network if your router supports it, and change default passwords.