Modern smart switches are pretty small (most of them are designed to fit into wall sockets behind plugs/light switches).
I personally think relays are a much more reliable than solid state switches and are very unlikely to fail in a dangerous way, and fully interupt the circuit, but they do have a 'click' some people dislike, and have a lifetime of 100k-ish switches, so for an application where you keep switching rapidly (e.g. not light switches), this might be a problem.
Ikea used Thread and Zigbee which are not Wifi, they use a mesh network and don't suffer from saturation the way Wifi does, in fact adding more devices tends to make the network more reliable since devices can route around failing or congested nodes.
I've had good experience with them in practice, but do be mindful that they share the 2.4GHz band with Wifi so in an apt building, you might run into radio channel congestion.
Personally I use smart home stuff for controlling heating devices and a few other key items, I don't think it makes sense to make every light switch smart, but technically people have done so and it tends to work all right.
I personally think relays are a much more reliable than solid state switches and are very unlikely to fail in a dangerous way, and fully interupt the circuit, but they do have a 'click' some people dislike, and have a lifetime of 100k-ish switches, so for an application where you keep switching rapidly (e.g. not light switches), this might be a problem.
Ikea used Thread and Zigbee which are not Wifi, they use a mesh network and don't suffer from saturation the way Wifi does, in fact adding more devices tends to make the network more reliable since devices can route around failing or congested nodes.
I've had good experience with them in practice, but do be mindful that they share the 2.4GHz band with Wifi so in an apt building, you might run into radio channel congestion.
Personally I use smart home stuff for controlling heating devices and a few other key items, I don't think it makes sense to make every light switch smart, but technically people have done so and it tends to work all right.