two-chords teaching: barre chords
That thing that is really hard.
Barre chords are the thing on fretted instruments that happen in some way on all instruments: it's the thing that sounds like crap until you build up the muscles to hold everything down and then it doesn't sound like crap anymore.1 These are the things that often make or break people. It's what makes the difference between continuing and quitting. It's that point where the oblivious parent asks the kid to stop practicing because they, the parent, can't deal with the sound.2
Now, if you are playing an electric guitar with low string action, some nice distortion, significant volume and a love of punk aesthetics, out of earshot from disapproving haters, well, you might not even notice barre chords are an issue. Barred power chords are your bread and butter and all that loud distortion will hide the fact that maybe, just maybe, you aren't actually holding all the strings down hard enough that they ring. But if you're like me — a general music teacher who, unlike the aforementioned parents, would actually rather listen to a room full of middle school kids desperately trying to get their ukes to do more than "chk", than talk about music and test memorization abilities — you've got to pep talk everyone through this "chk" phase.
People don't like to hear themselves sound bad. It's hard not to take it personally. It's hard to disconnect your sense of self from your sound. It's hard to believe that weird music teacher, when she says that this phase is normal, and that it will improve as long as you keep at it. Everyone wants a capo to save them from this embarrassment. A capo on guitar, of course, is really more for getting certain voicings that you can't get with barre chords (The G-chord and C-chord are the more obvious ones.). On a uke, it is truly unnecessary. There are only four strings and you have four fingers.3 No one wants to hear this, but the struggle is real. I simply say, "We don't have any ukulele capos."
My students encounter barre chords usually in the songs that they choose to learn. This is both motivating and frustrating for them. It depends on the student: some are motivated by the song to keep at it, some are frustrated by the physical difficulty and the bad sound quality and the song choice isn't enough to motivate them. Sometimes they switch to another instrument (piano, glockenspiel) to avoid the barre chords completely. But the barre chords will keep coming back. And I will keep doing the pep talk.
Other examples: On flute it's often getting a sound in the first place, on clarinet it's going over the break, on sax it's realizing that your sloppy sound isn't actually a good jazz or rock sound, on brass instruments it's getting your lips to produce higher notes… All instruments have their make-or-break phenomena.
This is definitely one of the narratives I abhor the most and wish more parents had more of a clue. Thankfully, my non-musician parents — maybe because they both grew up in large, loud families? — were able to put up with four practicing kids.
I would only use a capo if I wanted to leave the lowest string as a drone or something of that ilk — in a situation where I would be doing something fancier than simply playing traditional chords — and wanted a Db or D or something.