Changing your electricity or gas company does not affect the reliability or quality of your electricity and gas supply. When you change suppliers with Switchwise, you are simply changing your retailer - the company who sells or "retails" your energy and sends you bills. Switching retailers does not change your distributor - the company that owns the meters, wires and pipes and supplies the gas and electricity into your home.
If you are moving home rather than switching sometimes your power supply will need to be disconnected and reconnected by your local distributor. This is usually done to enable the distributor to put in a new fuse - for safety reasons this cannot be done with the power running. In such cases you would need to switch off your power at the mains on the day your power is to be reconnected so you would be without power on that day.