So much bullshit.
Firstly, in terms of PCB level repairs not being reliable; sure, if there's a hole in the motherboard, it was goused with Coca Cola and it was repaired by a monkey, yes. It is less reliable. The entire board is corroded, many components are hanging on by a thread... sure.
But 99% of the time they last and work just fine. The reason PCB level repairs are not done is not because they are less reliable, it is because you have to have a brain to do them - something most have stopped using throughout the new millennium.
Fast forward to 23:50 in this video. https://www.youtube.com/watch?v=885LDVit1Yw What is holding this stuff to the board from the factory just sucks.
Don't bother with Apple certifications or programs, they qualify you to unscrew the board before you hand it to me to actually fix it. They are worthless.
To answer the OP's question, how do you learn how to repair logic boards? You watch my channel. Listen, think, absorb.
https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAgtJug50pRNQv0