In general I would agree, but under peculiar, special, unusual circumstances, like a constant CURRENT switcher, rolling your own might be the only option. The simpel switchers are nice and simple, but benefit from GOOD pcb design techniques.

And I would never make an on-line switcher !

A friend of mine with a degree from one of the smaller institutions tried to make an argon -ion laser driver by trial and error !

He went through DOZENS of power mosfets having failed to realise that inductors have a nasty habit of saturating, when you wind "a couple' of turns on "an old television EHT core"

Don't do it !

Steve