When you say “flush”, it covers a range of procedures. Generally a “drain and refill” is not classified as a flush, and there are two of those, one where you just drain the radiator and one where you drain the complete cooling system.
Flush can mean that the vehicle is hooked up to a flush machine. These might also be called a reverse flush as the machine may force the coolant backward through the coolant system to dislodge anything that might create a blockage. I don’t see any harm in this type of flush, but I think it is unnecessary in most cases.
A flush done in the driveway probably denotes where the system is fully or partially drained and then water is forced through the system with a garden hose. Some people use an adapter that is permanently spliced into a heater hose, others just push the hose into the radiator. The adapter does reverse flush the radiator. The only harm I see in this is that the tap water may have some minerals that could build up in the coolant passages of the engine and radiator. Also, you will never get 100% of it back out so it could dilute the antifreeze some.
Another version is to put a chemical such as “Super Flush” into the radiator, then allow it to circulate for a specified time, then drain, flush out with a garden hose and finally refill with fresh coolant. The disadvantage again is that you may not get all the chemical out and it is very hard on the cooling system. I personally have had some very bad experiences with this kind of flush and would not recommend it under any circumstance.
If you follow the manufacturers recommendation listed in your owners manual for changing the coolant, that is a simple drain and refill, you will never need any type of flush. Once you are past the warrantee period and if you use a long life coolant, you can extend the drain period to 5 years or 150k miles. The new extended life coolants are very good coolants, far superior to the old “green stuff”.
Personally, I use the extended life coolants and have used them beyond the 5 year recommendation, well beyond, so I believe they have plenty of buffer built into their drain interval recommendation. I don’t recommend that anyone else go beyond those recommendations because I don’t wish to take responsibility if something goes wrong in their vehicle.