Engine flush is not a necessary thing to do.
Is all boil down to the car engine itself.

Personally, I did it once before.
I just gotten a 2009 Saga.
Slightly less than 50K KM, history? Not really known.
I had a very long long thought about using engine flush.
So what led me to using it?
Wanna remove all those sticky/gunky engine oil that look a bit bad.

Bought prestone engine flush at ACE.
Me and my friend started removing the old oil right after parking it in the workshop.
The oil wasn't light in viscosity.
Now, we poured back the old oil with engine flush.
Idle for 5 minutes. Some observations was engine sound a bit louder.
Removed the engine oil, now is a lot lighter in viscosity.
Is like water pouring out.

Conclusion?
I didn't felt much effect.
Cleaner maybe?
From the engine oil fill hole, is slightly cleaner.
No blue smoke or anything. Previous owner didn't torture the engine maybe?

Use it again? Maybe with a different brand.

I personally, wouldn't do it to an engine that isn't well taken care of.
But then is a bit dumb, if the engine is well taken care of, there wouldn't be a sludge or any problem.
Thus, the engine flush is unnecessary.