This discussion is about operations on 1 engine. Flap selection during a turn is a possibility on arrival. You have the a/c rudder trimmed out/in balance nicely with a stable thrust & speed. Boeing's advisory for correct rudder is level aileron control. You are now in base turn, start to extend flaps and by consequence need to slow done and thus reduce thrust. The trim/balance you had is now not correct. You are in a turn and the aileron control is now not so easy to assess. hence the difficulty to achieve perfect balance and hence the 'not recommended' comment.
I used to emphasise that, using KISS principle, it was easier to have F5 and stable speed before the base turn and thus there was no configuration change needed during that turn. If you do that on 2 engines why make it more difficult during an NNC on 1 engine?
To demonstrate this to students, who wanted to do it differently and delay everything, including being level flight, I would give an ATC descent & a speed reduction instruction; just after they started to execute these I'd give a base turn HDG. Not a shortened finals; distance was fine. With A/P in CMD they ended up in a right turn with the ailerons turning left and didn't seem to think there was anything wrong. I'd then point this out and the slip indicator and ask why they thought this 'unusual' behaviour was happening? The eyes opened somewhat. We'd then reposition to platform altitude, F5 and stable, and fly the base turn manually in level flight and constant speed. In fact it needs a 2-3% increase in N1% that should be applied before the turn and a squeeze of foot to compensate, then turn. As you roll out they are removed. KISS. Which did you find easier? there was a sizeable majority in favour of.