If you're watching a fextralife guide it's to make the game easier.
Why on earth would anyone watch a guide that offers no help to that effect?
You just... wouldn't watch guides at all.
I sometimes watch guides just to see what other people are doing, half the time I have no intention of doing anything they do and the other half of the time I only learn what not to do from a guide.
And this one is a perfect example of the latter. Fextralife suggests respecing Shadowheart out of her subclass but Larian very strongly suggest that doing so will completely break her companion story:
"Keep in mind that certain Origin stories are closely intertwined with their characters' respective classes. If you’re opting for a non-cleric Shadowheart or a Wyll without his warlock pact, this will undoubtedly influence your roleplay options in dialogues and the overall narrative for these characters. But their stories will instead be yours to create."
So I am absolutely NEVER going to respec her.
Then if you look at most of his party compositions, very few include Shadowheart. So as far as I am concerned that guide is an excellent guide of what not to do in BG3.