Wiring Issues
There’s no technical problem as long as you separately provide constant current to the bulb.
The switch then acts exactly the same as a remote that you holding your hand. It sends a message to the hub. The hub sends a message to the bulb. The bulb hears the message because it has current even if it looks like its off. The bulb decides how much current needs to draw request.
In most places in the US this kind of wiring is legal. However, there are some townships where it is not. They want turning the switch off at the wall to cut current at the light fixture on the ceiling. The reason is that they believe that someone who might want to replace the whole light fitting might just turn off the switch on the wall, start unscrewing things, and electrocute themselves. Again, most jurisdictions believe people will know enough to turn off electricity at the breaker but it is not to code in all places unless the visible switch turned off the current to the fixture.
I honestly have no idea whether it’s legal in the UK to have current always run into the ceiling fitting or not.
If you allow the switch to kill current to the bulb (rather than ask the bulb to turn itself off), things can get out of synch
If your local safety codes, or your insurance company, or your landlord, will not allow you to wire a constant current to the fixture that holds the smart bulb, then the problem is that turning the switch off kills the current to the bulb. the bulb cannot hear the next on command. It has to be turned on at the switch again before it can be turned on at the bulb. This can create a lot of synch issues and maybe rules that you set up don’t run the way you expect the next time because the hub doesn’t really know what state the bulb is in.
Power Outage Behaviour
In addition, many manufacturers of smart bulbs have them set so that after power is cut completely, when the power comes back on the bulb comes on at full brightness. That’s sometimes surprising to people. Whether or not it’s a problem for you just depends on your own household. You can still turn off all the bulbs easily once the power is back on. But it might wake up the kids.
A lifestyle approach–and voice to the rescue
If you’re OK with the behavior that occurs after the power has been cut, namely the bulb coming back onto full power next time, a lot of people do just address this as a lifestyle decisions. They will only turn light off at the switch when they know that it’s ok to Make the bulbs out of synch. They just deal with a synch issues that occur.
At my own house, we solved the synch issues by adding an Amazon echo. If you’re in the US, the echo/SmartThings integration is fantastic. It’s now the primary way we control lights in our house. Everybody likes it, including guests, health workers, my housemates. We almost never touch the switch although the switch is there if we lost Internet.
I know voice is not currently available in UK, so that’s frustrating, but long-term that’s another thing to put into the mix before you start investing in switches. If voice would work for your household, it can solve a lot of issues.
So it’s just one of those things. For some households the synch issue is a big problem. But for other households it’s no problem at all. So there’s a lot of variation.