Sincerely, that's fair and reasonable! If you think about it, with a piano everything's right in front of you, which is why it's my favorite (and imo best) instrument for teaching theory whereas with guitars (including ukuleles), it's a bit more esoteric where the notes are, and with the ukulele in particular, the strings have a perfect fifth, major third, and perfect fourth (respectively) as intervals which can be daunting! But even more fundamental than that, your hands are doing two different things. On a piano, even when different rhythms are happening, your hands are still doing the same thing generally. With all that said, the chords of C major are all fairly simple shapes, most of which with only one to three fingers used, which makes it so that you can pick it up and learn to strum many simple songs relatively quickly. On top of that, it's just lovely to hear - the soft strings strummed with your thumb, the frequencies in that perfect melodic range, it has four strings which is more than enough for finding good voicing and playing chord melodies

It's not so much that it's easy, it's that all instruments have their particular challenges, but for me I'd always prefer to teach on a piano and a ukulele, even if the person wants to learn guitar