Go to the Article: How to Make Great Espresso
I'd argue that (and I think James Hoffmann would too) that chanelling is often caused by grinding too fine. Grinding too coarse will cause a fast under-extracted shot for other reasons.
There's no need to be obsessed by 20 - 30lbs of pressure. Scott Rao (discusses that over 10lbs of pressure yields no noticeable difference in total dissolved solids. No need to tire your arm out on this one.
Channeling can be caused by lots of things. You could grind something perfectly and then poorly prepare the puck for instance.
As puck preparation is pretty important to extraction it might be worth discussing WDT and RDT to some extent.
Additionally, the water you use plays a key role in the flavor profile and potential scaling in the machine. Two more topics that might be worth discussing for people who are new!